The cast of Mythbusters is new to me, but wanting to RP as or opposite of real people isn't unheard of. I've seen more RPF than actual roleplays, but it's usually pretty harmless. Haven't you ever had a crush on a celebrity? I think roleplaying as or opposite of them is just an extension of wanting to play out that fantasy and probably self-insert an OC.
Although there is something a little different that involves celebrity/celebrity, and that has crossed the line with real people many, many unfortunate times. The most recent one that I can think of is Chris Pine and Zachary Quinto from Star Trek where a few people in the fandom thought it would be okay to show them their smut fanfics and ask if they were dating. And I believe there was also an incident with Rooster Teeth specifically, although I don't know that it was RP related, but a fan had accused one of them of being a closeted bisexual.
Really, though. As long as people aren't up in anyone's face with their writing, I stand by my original statement of it being mostly harmless. It's still a little weird though.