Rock Slide is also affected by Sheer Force.You would never run 252 attack EVs on camerupt. There are pros and cons to both moves.
Rock slide has more PP, but is weaker than ancient power without investment
Ancient power doesn't have a lot of PP, but is sheer force boosted, and with investment in s. attack, isn't too weak.
That said, yeah, Ancient Power is generally always your best bet as it means you can invest in bulk rather than Attack.