I posted about this awhile back. Most guitar players don't have as metronomic time as they think they do. This is coming from both sides of the the kit, as a guitarist -and- a drummer. The big lesson is that it has to be communicated to the drummer where you want things, or having enough trust that the drummer will put the groove and tempo where the band feels it and make it feel good if they're not locked to a click track or grid.
Here is a perfect example. This is the great Steve Jordan (currently filling Charlie Watts' chair with the stones) on drums backing up John Mayer. I don't think John Mayer would hire someone who can't play in time. But, if you listen to the intro on the sonfg below it is abundantly clear that Steve Jordan --slows down-- to put the tempo exactly where John is feeling it. If you listen to them playing this song 2 years later on the "Where the light is" live album, they play this song many clicks faster. This is a masterclass in how a great drummer "follows" an artist where they're feeling it and why guys like Jordan has played with everyone from Clapton, to Keef to Mayer and has been on more records than most.
Another key spot to listen for is around the guitar solo at 5:30 to the breaks back into the verse. Jordan decides to drop the hi-hat to stop dictating the time and clearly waits for John to re-establish the tempo before popping back into the groove. There's probably footage somewhere of this performance but I bet Jordan is watching John like a hawk to see his cues, his body language etc.
This is probably the most common thing I run into as a drummer that I always ask in rehearsals or on the bandstand if I am not using a click/backing tracks live. "Do you want me to put it where you guy are feeling it? Or do you want me to just hold it down?". Depends on the song, and/or style but there are plenty of bands that their entire time feel is how the push and pull against a drummer who holds down the center. And others, are defined by how the band pushes and pulls their time as a unit.
In other words, if the drummer is a good drummer he is speeding up or slowing down by choice, not because he can't play in time. Just communicate what you want and you'll most likely get it.