Slides might make sway when parked a little more pronounced as you move around (particularly if you're in the slide area) since it increases the lever arm somewhat. Leveling jacks help with this when compared with leveling blocks because they attach to the frame, and stabilize the frame against the ground, rather than having the support be through the vehicle suspension.
Weight wise, though, they don't influence the need for a leveling system; the chassis is fully capable of carrying the weight of the RV, whether parked or underway, and leveling with blocks works whether one has slides or not.
Levelers are a convenience I have no trouble doing without, and where payload is limited I would rather have the extra weight capacity for other things. Others may well have different priorities, of course.