Would something like that be useful or will most people want the capability
I am not "most people", despite my inflated ego 
But as you see from my reply earlier, I would have found that usefull.
My sweep used a millis() timer construct. For me - not a big hassle to program, but I know from many of the threads here, that it will be too much for many people to "do more than one thing" (ie let the sweep progress without disturbing other processing.) As I do not (yet) have any experience in accessing the hardware timers I do not know how much hassle it is to add that in the timer/interrupt code - or if it would limit the max number of servos.
(My sweep code has an advantage that I can do something synchronized with two servos. In an built in function one would need to change the sweep rate for each "ratio".)
The alternative is to let the sweep be "blocking", ie calling servo.write with asweep value will take the time it takes. I would not like a servo.updatesweep() to be called "often" for the sweep to work.