I may take some heat for saying this, but I think it's a true statement: 90% of all drummers' timing sucks. They can't play to a metronome. They can't play to a click. They may not even own a metronome. And that, of course, is exactly why they can't play in tempo. Okay, granted, music is supposed to "flow" a bit, and that is definitely evident in almost all classic rock (think Zeppelin). But no matter, as a musician, your job is to be able to keep it together - at whatever tempo - no matter what your instrument.
For the guitarist, I try to play with a metronome for everything: scales, chords, sight-reading, and even rehearsing my own music - especially for sessions. I challenge my students to have better timing that 98% of the musicians out there. The remaining 2%? Well, they better be drummers.