And how the heck do you aim those damn things? It's like trying to get a laser-pointer exactly on target with the first try - only lives are at stake!
This is what, in my eyes, makes the "assisted targeting software" theory so believable. I have one of those limited edition type 2 cobrahead phaser laser pointers ... and it's amazing how difficult it is to hit a target with it.
For the same reason, I'm much more willing to forgive the actors for their mis-aiming that necessitates the post-production beam adjustments. Well, that reason, as well as the fact that many of those phaser shots were probably done in five or six takes, and continuity people are only so good.