Drummers are the hardest position in a band to fill, and keep filled. A good drummer is hard enough to find, but a good drummer who is also responsible and committed enough to be in a working band is rare.
Many good drummers are just flakes, or crazy, or un-reliable. Hey, they hit things all day, right? Conversely, many responsible drummers are just not good. There is something in the raw talent to play the drums that also makes a person a little crazy, more so than other musicians.
So, if you find a good drummer who is actually responsible, hold onto that guy!
My band is currently on our 5th drummer. The first one was a jerk, the second one got arrested, the third one went crazy, and the fourth one got married and had kids. This 5th one seems cool, though.