Whether you see it as stupid or not doesn't really change the fact that it's well established in the alien mythos that they have acidic blood. And the folks that have made the films have even stated this. And ALIEN3 establishes that the corrosive aspects of their internal fluids are seen elsewhere in their biology (i.e. - when the alien spits in Murphy's face) - which is then seen again in AR. And in AVP Scar demonstrates the acidic properties of the alien's blood to Lex. And Bishop even makes the observation that the alien's blood oxidizes after death, completely neutralizing it.
Yes, Wolf's arm should have melted away (if nothing else he should have been burned when he rammed his wrist blades up through Chet's head... but nothing), just like the windshield and nose of the helicopter should have been damaged when Kelly shot the alien, just like the alien Wolf cut in to pieces with the whip should have melted through the roof of the hospital, just like there should have been deep pitting at the edge of the pool where Wolf killed the alien. The alien's blood was only corrosive when it was convenient to the VFX and budget of the film - which Colin even verified here in the forums.