It's true that most sport flying can get by with standard or lower powered servos.. the two factors to consider are flying speed, and surface area.
The smaller servos tend to have less sturdy internals.. gears tend to be made from congealed margarine, and can be stripped in flight with high loadings on the surfaces even at slower speeds.
Basically it's the users choice based on his experience as to how he flies his planes.
I have a lot of standard servos getting dusty now that I'm doing a lot of e-flight. Just don't need the extra size of the servo case.
And I've seen standard servos used in very large scale planes satisfactorily.
3D does require some consideration in servo choice. Small, ball-bearing and metal geared is probably optimum for the ailerons and elevators.
I see some very mauverable e-planes with very large surfaces fly very well with micro-servos.