At 3.5 doubles, if you have a decent and varied serve, and can simply put the first volley in play, you will win way more points on serve than you will lose. It is also a nice tactic to use occasionally in singles to keep the returner guessing -- but I don't think there are too many guys who are legit 3.5's that can win consistently doing this in singles against halfway decent 3.5 players -- there's just too much room to pass/lob a 3.5 volleyer in singles, unless they get to the net by approaching off a short ball.
S&V is probably how I earned the last .1 or so to get bumped to 4.0. I began playing a fair amount of 4.0 (all doubles) last year and while I win more points coming in than staying back, that's not saying much. The typical solid 4.0 has me at almost breakeven on my serve with good returns and the ability to totally destroy the weak volley -- that's no way to win matches. I would say that the % of S&V at 3.5 and 4.0 seems identical -- it's just that, as expected, the typical 4.0 is better at most, if not all, facets...