My apologies - the way it was written made it look as if the "inside CAS" referred to VFR/SVFR only, due to the lack of punctuation. (I have just spent a 9 hour night shift providing RCS in CAS however and I'm always picky between night shifts!).
RCS is still basically "must do". If the pilot gets to the stage that he must deviate from the clearance for flight safety reasons, then they've left the decision that they can't comply too late. However, it's not a situation I've come across as you normally issue a clearance which is easily complied with, and gives decent scope for deviation whilst still complying with the clearance as issued.
Going back to the original question - RAS for VMC only pilots. I've had it asked before, either by someone who obviously didn't know what they were doing (R/T voices are a big clue) or it's been asked for below MSA / RMC minimum levels (below which you can't be vectored anyway). In both cases I would say don't waste ATC's time and ask for it in the first place (And in both the above scenarios you wouldn't get any more than a RIS from me).