If a let was considered a valid serve, would professional tennis players be able to accurately hit the net cord and have the ball drop into the service box, often enough to make it worth while?
Would it at least be a commonly attempted on a first serve, with players opting for the usual safer second serve if they hit the net?
Or would the risk outweigh the benefits? Would players continue to serve as they do already, with a let being nothing but a lucky bonus?
I think that would depend on the player. If the situation you described was allowed, it would be akin to serving an ace. Current players do attempt aces by serving to the edges of the opposite player's service box, even though the chance of an ace is pretty low. However, some players (like Federer) do hit more aces than average, while some others are below average in that respect. Similarly, with practice, players could manage to hit the net and let the ball drop over on their service with increasing accuracy.
All this is completely hypothetical since the rule does not exist and it doesn't seem like it ever will as of now.
Have you ever watched Collegiate tennis or World Team Tennis? This is actually a rule - no-let serves. No player is talented enough to try and clip the net and have the ball roll over for an ace (or service winner) - that would require inch-perfect precision. Also, most players aren't particularly fond of the no-let serve rule because 1) they're not used to it and 2) shots that clip the net and roll over for a winner are considered "cheap" and most players apologize for winning points that way. Not to mention, the net cord produces unpredictable results depending on where the ball hits, what kind of spin is on the ball, and what angle it comes into the net at.
Here's an article that talks about the subject: http://www.nytimes.com/2013/01/20/sports/tennis/tennis-trying-a-no-let-experiment-on-serves.html?_r=0
