I personally like an instant re-spawn option on a timed re-spawn like CoD.  That would have to come with the instantly be able to be killed.  No safety time.

If you've ever played CoD 4 or 5 the respawn timer is prevalent for modes that it's needed in.  And not in modes like TDM or DM.  There is a timer, but you can hit "square" at any time to respawn. 

Hey does anybody know what the cross above the players forhead means right when they spawn???

I definitely think the respawn timer keeps the game a little less frantic by making sure there's not always a full map, but sometimes it gets really annoying.  The times that I find it to be unbearable is in matches with only a few people.  also, i'm not an awesome player and i find that many times (especially on ranked matches) I'll have to wait 20 seconds to get in and then after only a few seconds of game play i'll get killed and have to wait another 20, so that i'm rarely actually playing.  So like others, i'd say that there should be a timer on some games, but not necessarily others.