Why include them in general? I'd argue Veteran Space Marines are just as important as Scouts, Terminators and other aspects of the list. Lots of Chapters have special groups such as Honour Guard, Tyranid Hunters, Pyre Guard, Sword Brethren and Wolf Guard – not to mention generic 'Veterans' from the older lists, and Sternguard from the newer.
If you don't want to have a specialised Chapter list and prefer a centralised simple list to cover all Chapters, a Veteran formation gives the nod to these. Finally, a differentiated Veteran formation is a good way of allowing people to include old models they have, and provide a visual centrepiece for an army.
Why the specific rules I've suggested? Land Raiders don't seem to be hugely popular, but a small, tough, and easily-hidden formation that combines infantry and vehicles would be a nice way of including these great models without changing the points values/stats for the rest of the list. The two unit types (Veterans and Land Raiders) complement each other in the background, too, and differentiates them from the Tactical Marines formation (the smaller formation size helps here).
I thought
Thick Rear Armour would be a neat way to represent improved morale and adaptability over and above Tactical/Assault/Devastator Marines, and provides a link to their Terminator-armoured Brethren. In-game, it should allow the player to use their Veterans aggressively: to plug gaps, be ground-based objective snatchers, and generally act heroically, something I think a ground-pounding Marine list lacks.
