If you're a small enough studio that you don't need the post-production features which are only found in Pro Tools, and you're also the responsible for the final deliverable, then I'd agree with you there. I guess you can mix with whatever you want. I imagine some guys who really want to fly under the radar and don't aspire to do any commercial mixing work could somehow mangle Studio One or Ableton into doing mixing work that neither were designed to do. But it would be so frustrating and limiting that I don't see the point in not learning Pro Tools or Nuendo. Again, a DAW like Studio One is just a bad choice for anything requiring a higher level of features than a music mix. But if you're doing post, and you absolutely despise Avid, then since both Pro Tools HD and Nuendo are way out of the price range for a studio that small anyway, my suggestion would be to buy a copy of Digital Performer for $500 and at least give yourself a fighting chance.
I'm not trashing Studio One. Its a very good DAW, but only when used for what it was intended for. Same with Reaper. It is not a viable alternative to Pro Tools HD for anything beyond a stereo music mix.