It's always an "in case shit goes south" mentality. Like if a guitarist walks in and ABSOLUTELY MUST have his gain dimed on his spider III for "his sound". lol
Or in rarer cases where a band wants an amp I don't have, but that's happened once. I don't get it as a main plan of attack either.
Yup...for a studio setting, especially a commercial studio, it's necessary to have all kinds of options and some back-up plans, because you never know when the artists or producer will change their minds, which you can not read ahead of time.
I'm mainly looking at it from the home-rec perspective where most guys are recording their own music, so mind-reading is not needed, and IMHO, I just believe it's better to go with a more defined production when at all possible, though there are those rare times when you might want to lay down tracks just to see where things go, and then worry about how to make it all cohesive later on.
Talking Heads recorded some of their stuff, like the "Remain in Light" album, in that manner. Just compiling a huge number of tracks without any planned production...though I'm sure they had some ideas...and then during mixing they decided which tracks were going to be used and which would be discarded.
It's a fun way to experiment, and you might come up with something you never expected, but that was done for all the tracks, not just "guitar tones".
My own approach to "guitar tones" is pretty simple/straightforward, and I usually prefer to have my tone at the time of tracking, rather than deciding later.