Anyways, if the other team is spawnkilling yours, it comes down to two basic scenarios.
Either it's a lone spawn, isolated and undefended due to an in progress/failed base move, foolish building, or something along those lines. In that circumstance, it seems reasonable to to me that the team getting spawnkilled deserves to be punished for such a mistake if the other team team is willing to oblige.
If you are being spawnkilled during the last few minutes of an all out base assault, that should also be fine. If you are being spawnkilled as you exit a base spawn, that usually means that the spawn is under attack, will probably be destroyed shortly, and the other team will win rather soon.
A few extra seconds as a dretch will probably not be enough to stop marauding lucy suits from destroying your base. However, a few extra seconds for a dretch or granger might be enough time to escape the base, resulting in a long, drawn out bug chase or egg hunt. Most players don't seem to enjoy these very much.
A few extra seconds as a human won't be significant either. What is a human with a rifle or c-kit going to do? Fire his rifle at a tyrant until his invulnerability wears off and he gets chomped?
Also, spawn invulnerability will add significantly more "WTF?!?!?" moments to Tremulous. "I hit that dretch will a full charged lucy blast and it didn't die! HAX!!!"