Chris/Matt,
I suspect the reason we see so much use of the water carry on par 3s as opposed to par 4s and par 5s is because it is seen as more fair for the less accomplished golfer. Everyone starts out in position A (or at least the same position) with the ball on the tee, etc. When you do this on a par 4 unless it is a very short par 4 some golfers may be forced into playing it as a par 5, especially if they don't hit their drive well. Then you have the factors of people playing from rough, behind trees, or even out of a divot hole if everyone is laying up to the same place.
Its only worse on par 5s, plus you take the chance of going for it in two out of play for many golfers who would otherwise have it....the "I can get there with a perfect 3W so what the hell" guys, who feel cheated if they bust a drive and then realize the have to lay up because there's no way they can carry that water.
Doing it on a par 3 is a "signature" shot that everyone plays gives people something in common to talk about. Everyone has played the shot, but on a par 4 or par 5 everyone played a totally different shot from a totally different place, even if they are of similar skill and hit the ball similar distances. Look at us GCA members who might like to think we are above falling for this, but what two holes had the pictures posted and has had the most discussion in this thread?
Not that I agree with this, just trying to suss out the logic behind it...