Rear-focusing scopes can be more difficult to operate, especially when prone, and front-focusing scopes (i.e., adjustable objective or AO) are ALWAYS more difficult to operate, and often have issues playing nice with flip-up covers. Having the parallax adjustment located as a turret on the adjustment ring (where the other scope controls are located) is a nice convenience that can be quite important for a military sniper or sharpshooter.
Having said that, some bargain scopes (the Tasco SuperSniper comes to mind) have problems with side-focus models that aren't present with rear-adjustment models. Manufacturing a system that can move lenses such small distances with absolute repeatability and reliability is difficult and expensive, so compromises have to be made to offer this type of feature at a lower price-point. Just something to keep in mind.