Standard spawns give you like 3 second invincibility time. The problem is that, for the most part, people like to pick weapons or spawn in a special room. So the invincibility is used up before they even get out into the field. But if they were just given artificial spawn invincibility when they left the room, that would have it's own problems.
On a related note. I think it would be could to have some sort of power source in a base, that would supply it's soldiers with various shields of different strengths depending on how far they were from the base. So the difficulty for one team to push another back into their base would increase the farther they got. I think a lot of neat strategies could come out of it, however, snipers would become way too powerful.