there are good reasons they don't come in after 'every'
- It's predictable.
- It's predictable and with current techniques and strings can be passed effectively.
- Current pros* are not as good at volleying as the late 90s generation due to lack of S&V on tour and limited grass season. Which players can hit an approach volley or half volley and then finish off at the net in a couple of shots? Anyone can hit a put-away volley on a floater.
However, volleying and in some cases serving and volleying can be very effective if used sparingly. We saw this in the Marseille ATP tournament int he last two weeks. Not just Llodra (vs. Del Potro in particular - awesome match by the way), but also other players like Simon, Tsonga and such coming in during points to put pressure and surprise their opponents.
It can be very effective in tie-breaks or on key points.
S&V is like a drop shot or running around your backhand to hit an inside out forehand. If you use it too much, without setting up the point the better players will expect it and adapt their game to counter.
I think players like Murray, Berdych, Del Potro and Gasquet could use a bit of volleying in their game. Would give them the dimension needed to raise their game; round it out a bit more and give them a weapon they can rely on when things get tough for them. Nadal has been volleying a lot more in the past couple of years, although those are more like put-away volleys or swing volleys. Maybe all of the doubles he's playing now will help him incorporate that more so he doesn't have to grind it out as much.
* The exceptions are few, the older crop of S&V people like Llodra and Stepanek, who also play at lot of doubles. Or all courters who played in the previous generation like Federer.