Some X-men allies, Rockslide included, don't seem to be that good for the cost. 4 is a very high cost to merely hit twice for 3 and go down against a villain attack or swing one last time. But you have to remember that a lot of the X-men allies are not great for the cost to begin with. The reason lies with the impressive amount of synergies they have with things like the Training upgrades and Danger Room. Insert Rockslide alongside these 2 and you suddenly have a 9 HP ally that can deal 12 damage over 4 turns AND take a villain hit before finally being discarded. Doesn't even need a med team for that value. There is no easy-ish way to do this with non-Xmen Protection allies with other traits.
Another great way to play Rockslide is with Mutant Protectors, in which case you cheat him on the board at 1 cost and defend with him. At 6 HP he can comfortably take a villain hit without getting removed most of the time with the added bonus of 1 Retaliate. You can then use him to attack or defend again the next turn, which can be quite good at 1 cost.