How do you expect it to get there without partner showing spades if he has them? If partner has 4+ spades and enough to bid, he'll be negative doubling over anything from LHO. You might occasionally miss a 5-3 fit, but the amount of time that happens AND the opps bit to 4 of a red suit AND it would be right for you to bid 4S over that AND you aren't able to do so aren't particularly large.
The suit that it WILL be difficult for partner to mention is clubs. If it goes
1S (3H) P (4H)
?
Then there is a much higher risk that you missed a huge club fit than of you missing a huge spade fit if it goes:
1C (3H) P (4H)
?
and it's the same over 2H.
Edit: This is not to mention the main downside of not opening 1C, which is obviously that you wreck your own constructive auctions by misinforming partner about your distribution.
Edit #2: Also, while I don't know your system, that 2S bid doesn't sound particularly descriptive. A hand like:
QJTxx
xx
xx
AQxx
and this hand are miles apart.
Last edited by ChrisV; 11-26-2008 at 06:34 AM.