I think the idea is admirable... but I forsee two problems.
1) If you're talking about letting everybody see what spawn is flashing, then you're also showing exactly where people are coming from. If dar is down or planes are below radar, you can't tell which way planes come from... so why should spawns show exactly where the defenders should go to kill the attackers?
2) If you're talking about JUST letting the attackers know when an enemy is in the area of THEIR spawn, then I think it'll be going off whenever you're attacking that base. It won't give any clue that its spawn camped or anything. Also, it will give away any mission taking off from that base, an equally poor gameplay element.