I grew up around Ferguson and Massey Ferguson tractors. Harry Ferguson invented draft control. Harry pitched his developement as highly usefull with "soil engaging implements". That includes a 3-point disc. From my experience, using draft control with a mounted disc is more of a case of need for TRACTION CONTROL than of depth control. Even in plowed ground, 9 times out of 10, the biggest issue with mounted disc's is getting it DOWN to the desired depth as opposed to keeping it from working too deep. In most cases, position control can be used with great results to "gauge" a disc if operating depth is greater than desired.
I have used draft control with my 3-point disc on a few occasions. Hills and too much rain make for lousy traction conditions. Draft control helps in conditions like that. MOST conditions, from my experience, it doesn't make much difference which method I used. (pos. control vs. draft control) BUT, you have to use one or the other and draft control has it's subtle advantages, so, I USUALLY use the draft control.
Now, before I get everyone all in a dither, I generally use a tractor with more than adaquate HP, plenty of weight/traction, and a disc that is just a bit SMALLER than what the tractor is capable of handling. If you're trying to maximize the implement size you're pulling, draft control is probably more critical. I follow the logic that smaller implements/higher ground speed is more efficient. (To a REASONABLE degree)
Long and short of it, I don't think there is a "carved in stone" answer. Try both and see which works best in your situation. There doesn't seem to be any negatives to using draft control, and it does have occasional benefits. All things being equal, use the draft control.