I'm for No-Spawning or Formation spawning as suggested by E&C. (PS all of the below is IMHO)
A number of people have brought up the concept that spawning is more Nids joining the fight and becoming organised. I am against spawning representing this and that this concept is not needed for Epic in normal games.
1. Most Epic games are about a grand assault, a major scheme by both sides, to take obj. or kill a target, or even hold the line. The Nid's and the Hive mind will be fairly organised, and in theory would not leave potential troops lying about in the middle of a battlefield just in case a larger group meet it. They would return to the hivemind/synapse as soon as they can, not linger, not without purpose. The Nid's doctrine is fundamentally one of recycling and not wasting. Leaving troops out there doing nothing is waste, especially when they could be broken down into new stronger Nids.
2. Carrying out an assualt like the Nids do in dribs and drabs is pointless, any decent defence will wipe it out. (I'm sure many of you have seen this in EPIC games and even ensure you don't go half-assed into something) Nid's wouldn't go half-assed anywhere, all out or nothing. Holding back isn't likely to happen. Hivemind says that is what is needed for the task, do it. Allowing for some contigencies but not in the same way a guard army might have multiple lines of reinforcements.
3. Even during an assault in background the Nids are not described as fast in the sense of DE or Ravenwing or Eldar or Marines. Large forces need time to prepare and get in combat states. Some specialists like a Drop insertation, or Ravenors etc will be good at this, but not the whole horde. Meaning reinforcements are unlikely to just appear, as with other points it is unlike a Nid to linger without a plan or purpose. (Genestealers and lictors exceptions due to purpose)
4. An Epic board is only still a small section of an invasion. Like 40k can be seen as part of a Epic engagement. There is always a bigger picture. The idea that what is on the board is all the resources an army can use to defend or attak this section of the campaign is there or above via ships etc. Everyone has seen Starship Troopers and background for Nids in some form, they attack together and rarely hold forces back. The force on the board is what is available at the time and that is this 'Wave' of Nid's. If you lose they send another.
Individual Formation Spawning/Repairs makes a lil more sense if people want spawning/continous assault feel. The idea that horde is much larger than the models represent/footprint represents could be tailing behind.
Although I would rather the only type of spawning come from spawning creatures, that spawn a few on a marshal action, be it like Void shield repair or just a few D3 or whatever is found to be acceptable. As these units spue out a new units from the things they harvest and recycle. So it adds a bit more horde and some sort of endless feel, but not too much, of a formation suddenly appeared, but a few more gaunts are there now.