I just thought of a reason why continuous beam fire could fall out of practice. Well, two reasons.
The first is overpenetration. That's a problem with real-life bullets; they can pass through your target and hit innocent bystanders behind, or pass through a wall and hit someone you didn't know was there. A phaser on the higher settings poses the same danger; as material is vaporized/disintegrated, anything behind it is in jeopardy.
The other is the general unsteadiness of the human hand. Try pointing a laser pointer across a large room and keep it steady. You'll find you're involuntarily moving the beam over a surprisingly large area unless you work very hard to be still, which isn't going to happen in combat. The further out the beam travels, the larger arc is created by the beam.
I just thought of a third - if someone trips (or dies) and falls on the fire button... yikes. Might take out everything in a half kilometer of the beam direction before someone's able to roll the body off the button. Bad idea for the same reason light sabers turn off automagically when dropped