Volcarona's a hell of a lot scarier if you lose Rain to Sun or Sand, just sayin'. Since it can actually kill something without Hurricane. And tank a Special move while using QD. And that nice STAB Heat Wave. And Fiery Dance.
In rain, yeah, Vivillion is probably doing better since it lacks a handicap, and has a much better array of abilities than Volcarona. Shield Dust also lets you dodge a flinch rom Fake Out, that's noteworthy imo. I never use Rain, I've been all Drought since 2011, but I have used Volcarona. :P
Chansey would get any Egg moves the Chansey knows, yes.