1) Removing different common brood units from the front of a swarm will probably change the makeup of the swarm, which can significantly affect the way the swarm works. (This is also true for Orks and any other race where there are several different unit types within a single formation).

Edit:- just reread your reply, And I think I can see where you may be mistaken about the 'book' method.

You place the template (possibly with some discussion etc), and then you determine the number of infantry, vehicles and titans there are under the template and throw the relevant number of dice for them. No distinction is made for the types of infantry or character while dicing for hits or allocating them front-to-back under either approach.

The only difference is where the hits are allocated front-to-back; under the template or not. It is this aspect that can cause significant differences. In the case of the 'Nids this raises the possibility of killing a Warrior in the middle of the formation, which would not occur if the hits are allocated to the edge of the swarm. For Orks it potentially 'misses' Grots placed to protect warbands from the effects of shooting and assault, potentially changing the way the formation behaves later in the game.

However, I agree that for the vast majority of Races and formations the difference between the two approaches is negligible.

_________________
"Play up and play the game"

Vitai lampada
Sir Hemry Newbolt

However, I agree that for the vast majority of Races and formations the difference between the two approaches is negligible.

Agreed.
With the changed method being quicker and easier.

Also Grots and Expendable troops are factored into an army as their 'trait', something to make them more durable instead of just better Armour Saves- so why do Barrage weapons get to ignore this intentional 'meatshield' effect, where every other weapon must go through it?

It just seems unnecessarily fiddled, just for the added hassle of sniping important units.

_________________
"Play up and play the game"

Vitai lampada
Sir Hemry Newbolt

As the Eldar player in question, perhaps i could provide a little more information on the game being described.

Terrain:  Looking at Chroma's bat-rep, i'd say that we had as much terrain, it would certainly have been a challenge to lay any more down.

The centre of the board was dominated by a moderately large cityscape and 2 big hills, one of which was craggy and provided a 5+ sv forming a narrow series of valleys.

I deployed my objectives, just in Jeridian's half of the table, just over 30 cm from the board corners (Alternately with my opponent of course).  This meant that my objectives were deeper into my half and there would likely be a central one to be replaced with the webway gate and also that my opponent's garrison's would be split up and very far from any support from each other and the bulk of his army.

During the first turn i picked apart the isolated garrison units - concentrating on supressing and subsequently destroying the biovore thingy unit with artillery and air units.

Using my first 2 activations on aircraft and the void spinner meant that as well as pouring fire into the isolated garrison unit i could wait a little to see how the Nids would move the bulk of their forces.  The objective set up kind of forced both our hands really.  The nids pushed en-masse down the centre into the narrow valleys whilst i spread out into a crescent fm (My tactic of choice tbh).  Making the most of the speed advantage i clustered my forces close to the main nid line of advance.

At the start of turn 2 i won the initiative as i gambled (Fairly safely really) i would and after loosing a barrage with the void spinner, retained and initiated an assault on the large fm of nid infantry advancing down the narrow valley, parallell to the one the Dominatrix unit was marching down.  Using the commander ability and my speed i managed to get my aspect unit, a falcon troupe and a powerfist Phantom titan into engagement range whilst avoiding supporting fire from the Dominatrix unit.  

Well needless to say it wasn't pretty with nids having little to no FF power.  The 3 units then moved backwards their full movement to avoid engagement based retribution from the Dominatrix fm - which followed the eldar and loosed a really nasty flame template weapon causing many, many blast markers on the aspects and coming horrifyingly close to breaking them.  This however put the Dominatrix unit into the same valley of death as the - now dead gribbly fm had previously occupied.  Using a left hook maneuver with my flanking force i marched a SS to the other end of the valey and deployed guardians from the webway catching the nids in a deadly crossfire. Jetbikes and falcons moved up into supporting fire position with the main nid advance completely surrounded. The Harridan engaged a falcon unit but found itself within supporting fire range of the Phantom and was promptly beaten, although the falcons in turn broke and ran.

Turn 3 and i summoned the Avatar towards the rear of the valley of death.  Again, winning the initiative i opened fire with the void spinner onto the tightly packed nids, retained and sustained fire with the Phantom who was less than 30 cm away from the dominatrix and put 6 wounds on it and then retained a second time, using the avatar's commander ability, sending the guardians and avatar into assault with the nids.  With supporting fire from 3 units (Jetbikes, falcons and SS)set up precisely for this purpose at the end of the last turn and the avatar's awesomeness the nids suffered horrendous casualties and broke and fled to be harried by strafing runs from the Vampire in subsequent activations.  

With the bulk of the nid army dead or fleeing Jeridian really had very little left with which to activate and mount an effective comeback.

What won this for me i think was the superior maneuverability of the eldar, crafty objective placement and an almost garaunteed first activation.  The skimmer issue was an amusing (For me anyway, lol - sorry Si!) sideshow but IMO wasn't really a deciding factor, although i would definately have taken more casualties during engagements, i'm pretty confident that i had stacked things very much in my favour.

It's worth noting that i've had quite a bit of experience with Eldar now and am really starting to find my feet, whilst this was the first outing for the nids.  I wish we'd taken pictures of this battle TBH as i do find Epic battles quite hard to describe.

I can definately see why Eldar are such a frustrating army to play against for nids.  The combination of high speed, excellent FF values and almost certain first activation potential makes eldar very well suited to taking on nids, especially an inexperienced Nid player (Although i'd never fought against nids either).

All that being said i did enjoy the game and my opponent was a good sport throughout and using his guard in the next game gave me an extremely tough game that by rights he really should have won (I think his dice hate him, lol).

During the first turn i picked apart the isolated garrison units - concentrating on supressing and subsequently destroying the biovore thingy unit with artillery and air units.