Let me take a wild guess.
You're a quite a "belligerent" and "unnecessarily confrontational" person.
I'm not confidently advising anyone to do anything. As I specifically pointed out, I do not know why one should not install Rosetta into Mountain Lion and I added that perhaps something had changed in the OS - e.g. Lion is different to Mountain Lion.
What is incontrovertible is that FCP 6 works on 10.8.5. So if installing Rosetta on Mountain Lion has some catastrophic consequence, perhaps the solution would be to backup your work onto a separate device, reinstall a clean version of Lion, run Rosetta, install FCP and then upgrade back up to Mountain Lion.
A lot of people seem to want to find a solution to this issue and are left feeling that FCP doesn't work on Mountain Lion. That is patently not true. Rather than trying to prove how clever you are by being antsy to well intentioned strangers perhaps you could be helpful and suggest clever, lateral thought ways of getting round this impasse.
If on the other hand you just want to be sarcastic and rude, be my guest. I have FCP working just fine. I'll leave you to sort out all the questions of those who don't.