Open Mind
I've played in a couple "garage bands" (or should I say "garbage bands") and never had a drummer who could hold a beat. The keyboard player and/or bass player would always have to try keeping him to a somewhat uniform tempo.
Then I was talking to my son's music teacher the other day and he said that it's very common, especially among amateurs, for drummers to have a lousy sense of tempo. What's your take?
Open Mind
Every instrument has some special fundamental physical property that is unique to it. For instance, if you want to play the guitar, your fingers (fret hand) have to be able to actually fit on the individual strings without damping others. Every newbie on a guitar is going to screw that up, but some players will never, now matter how hard they try or practice, be able to properly press the strings. It is just not physically possible for them to do so.
Same with drums. I can sit down for 5 minutes with anybody at a drum kit, listen to them, and tell you if they have potential. I don't mean potential to be a prodigy or the next Neil Peart. I mean potential, that if they practice enough, they will be at least OK and can keep a proper beat. I completey disagree with the music teacher (I'll lay $10 the person isn't a drummer primarily). Tempo isn't taught to a true drummer. You feel it. Always. I feel the rythms of everything. If an amateur (excluding the very young) can't feel a basic 4/4, they need to not be a drummer. I have seen countless kids be allowed to be in percussion in school when they should have been encouraged to try a different instrument. I know that sounds harsh, but it has been my experience. Some people can't adequatly play any instrument. That's OK. We're all different.
As to your experiences with garage band drummers, I've been there, done that, got the T shirt. A percussionist behind a drum kit - a drummer - in that circumstance has one and only one primary duty: keep time. Unlike in an orchestra, there is no conductor. It is the drummers duty to keep it all going. If the drummer can't do it (because of lack of talent, trying to be too complicated with the part played, or too much beer), then that person needd to not be in the band. They can roadie or something.
Do this..the next time you see a noticibly good (or bad) drummer play. This is one of the tell-tale signs. Watch their hi-hat hand. If they are right handed, that will be their right hand. Watch them playing 1/8 notes on the hats. You will always, always see a good drummer as fluid on the hats. The stick will caress the cymbal. It will be like watching silky waves. A bad drummer will be like watching a robot as they "chunk, chunk, chunk" away trying to beat the cymbal into submission. It is very hard to describe, but if you look for it and if you are exposed to a drummer who is good and one who is really, really bad, and can compare, you will know right away what I'm talking about.