Yes, it's common to reamp guitars, but that is beside my point. My point is that experienced artists know what tools they like to use to get the results they prefer. An engineer laughing in someone's face over a mic suggestion, let alone the most common mic ever used to mic a guitar cab, is awfully dramatic, and I wouldn't want to work with any engineer that did that. That's beyond unprofessional.
I've never seen a session where anyone's equipment selection/suggestion wasn't taken seriously, and often, yes, they defer to the engineer, but only after agreeing that it's the choice they want to make. The bottom line at the studio is- You paid for the session you get to do what you want. We'll offer our opinion, and explain why we think it might be a bad choice, but they're the customer.
In my limited experience with VO sessions, no, I've never actually seen the talent have a microphone request. If I didn't have that mic, and couldn't rent it, I'd ask them if it's absolutely necessary, but again, I wouldn't want them to show up and immediately have an issue because we didn't have what they asked for.