^ This ...
I bet a lot of the bands that don't rehearse could sound better if they did, and that their performance can be a bit hesitant as no-one is quite sure what the others will play all the time. At a bare minimum I think you need to rehearse beginnings, endings, breaks as those will reveal how "untight" the band is.
We're talking covers, performed by paid professionals, so I can be *absolutely certain* that the others will play exactly what's on the reference recording, including some fairly complex material, with any changes as per the BL's emails. "We'll end it the way they did at Montreux in '78... here's a link" or "I'll repeat the melody line twice when my solo is done." No need to be staring intently at each other and, to address something someone else wrote, I'm perfectly happy if people ad-lib their fills as then I'm amused since I am listening to and enjoying what my bandmates are playing.
Why would appropriate fill, meaning one that's in time and in the style, throw anyone off? Heck, I sometimes gig unrehearsed with a jazz/prog wizard of a drummer who *deliberately* tries to trip me up and while sometimes he succeed it's a lesson well-learned that I should be counting for myself and that I'm lucky to share a stage with someone at his level. Great learning experience, and it's never the type of thing an audience will notice so no harm done.
I have done this more times than I can recall, often with nobody onstage ever having played together or even met before, including a drummer who doesn't even own a kit. When nights include music that had the token one rehearsal before 1st gig and songs that we didn't have time to rehearse or that were added last-minute, inevitably the ones that weren't rehearsed at least as good and often better than the ones that were.
If players you trust will vouch for other players, you don't need to meet, baby-sit, worry about, or rehearse with them as long as there are sheets or a reference recording and any necessary notes sent around. Originals or covers.
For those of you who can't believe this is possible, or common in the pro leagues, I can relate. I used to think it was all bluster too, until I started getting gigs with pros. There just isn't time at that level with most guys in multiple active bands, and the BL usually doesn't want to pay for people and a room to rehearse. Same with "originals" (everything's a cover if you didn't write it) even on national broadcasts.... meeting the drummer in the green room and discussing the audible cues that'll come up is probably more the norm than the exception.
Another common thing is for the name artists to hire most or all of the backing band in any given city, especially for festival tours where dates are scattered and distances are huge. No need to pay travel, hotel, per-diems, etc. for a whole band. They arrive in the city, and it's uncommon to have a rehearsal since they're doing press, etc. Maybe some or all of the band gets together with the local hires, say at the hotel or backstage to go over a few unusual bits or things that have changed since the album, what to do instead of the fade out, etc. That's about it.