Although I think it's a good idea in general, especially for hybrid classes, I have some reservations.
PuGs, for instance, are often picky. My guildie over at Zen Tanking has covered the main points, but my main concern isn't that I won't get invited to PuGs as a "pure" DPSer, but more that PuGs won't invite me if I'm not the top DPS spec.
Here's my issue:
In BC, there was a guy on our server running Hyjal (and later, BT) PuGs - and this was before the huge nerf. I whispered him my stats, and he rejected me.
I don't give up easily, so I managed to convince him that I stacked Agility (even though I was BM and not Surv) because it scaled with Blessing of Kings, and that was why my AP was low (it was true, but it was also a desperate gamble to get in).
It paid off. He invited me, I did decent DPS (not top, but not last either), and I was OK for all of his future raids because he remembered me from previous runs.
At least two of my guildies got rejected due to stats for the first few PuG raids (they were invited to later runs); one of them as geared as I was, with similar DPS.
My guild won't mind which two specs I choose, but PuGs have already extended from asking for spec to asking for stats; what's to stop them from asking for talents as well? Is everyone going to expect that at least one of your chosen specs is the latest FotM "cookie cutter" PvE spec?
I hope not. Fingers crossed the day that I decide to respec into a cookie cutter spec just so I don't have to justify my talent point selection to people never comes.