I do see the merit in keeping your set more creative.
I agree with you, I just wanted to mention that point. You cant always switch into Ferrothorn, which you had mentioned because if it Power Whips on switch then dugtrio would die.
The point of Rock Slide wasn't as much for the secondary effect(which is nice) but moreso for the increased amount of PP. Having a move with 8 PP as your only attack could easily detract from your sweep, as you could easily be forced into Struggling. Rock Slide would help to alleviate this problem.
EDIT: This is for the slash, not saying to use it over EQ