If the other team can get someone to sit in your base and kill all your players as they spawn, it means two things:
1. The map is not incredibly designed.
2. The other team deserves to win that round.
So yes, it's annoying, but it's part of the game. Giving newly-spawned players invincibility would be make it very tough to defend the other team's base when you're trying to keep them from capping.