People are using this trick to win PVP, my suggestion is to give players minecart flag, he stayed here for a while w/out being killed.
30 seconds is a great time in my opinion. I was actually the one to suggest it... But it really is needed. I used this example in my original suggestion, but captures the problem perfectly: I'm fighting two other people. I kill one of them - it's 1v1 now. It's a close fight, and all of a sudden the previous player rejoins and picks up a double kill. It's not fair. Which is why I suggested a change. So, you're suggesting 15 seconds? What you need to realize is, with good gear, fights can last a lot longer than that, allowing the dead player to rejoin immediately. Besides, 30 seconds isn't that long to wait.