Sure, but there are tons of Pokemon that are x4 weak to Rock, so even if it's just a measly 70 power, it's still enough to blast them. Like Volcarona or the millions of part Flying types (useful when conserving PP in-game, which Power Gem has plenty).
Also, remember that there are several Ice type Pokemon that are neutral to Flash Cannon. Like those pesky Water/Ice types and Rotom.