Honestly, in all the gigs I play (just under 200 so far for 2010), I have never needed anything except the following:
- Back-up snare (never actually used, but if the main snare head goes, don't want to waste time changing it).
- Extra sticks (duh)
- Advil, Tums, Immodium, Band aids, etc
- Duct tape - If I break any head, I can tape it up and make it sound decent enough for the rest of the show. I don't carry extra heads unless it's a multi-night tour.
- Drum key - I tune to different rooms and situations.
I've been drumming since I was three and have never lost a felt. I carry extra ones, and it seems common here, but I just have never had it happen. Of course, now that I said that, I'll lose three or four by the end of the year.
I also find it's wise to keep the following on hand, just in case:
- Pack of your guitarists favorite strings
- A few pics
- Extra mic, mic cable, mic stand, etc. Mic stands may break more than any gear other than sticks and strings.
- A quarter inch instrument cable (standard guitar/bass/keyboard cable)
- Electrical tape - you just never know when you have to run wires in a bad place, or are outside and need to tape down setlists, or whatever. I also have it in two colors (Purple and gold, for my Ravens), so that if someone borrows my gear, I mark it with purple/gold/purple tape so they know that it's my cable or whatever they are using, and I get it back at the end of the night.
You just NEVER know who is going to forget something small, or break their last one. In my more professional bands I haven't had to worry about this much, but a lot of times in the past I have played with guys who forget EVERYTHING. I used to go through about 8 picks a month playing with dudes who forgot theirs, or didn't have enough and ripped one. For the low price, it's worth having that one extra mic cable, instrument cable, pack of strings, and a few picks.