Depends on where you go to school I guess. If the employers who come to your school know that only the best students get RA gigs, then that's one thing. But I don't think that's the case at most schools.
Up here, there are definitely two types of RA's. There are the types who book the class and are asked by their professors to do research. Those folks typically acquire their positions early (like after Fall grades come out) and just don't want to deal with the stress of trying to find a summer gig.
But then there are the students who don't do so well, simply fail to find jobs during the Spring, and then get bailed out by career services at the 11th hour. I think when a firm sees a mediocre GPA and an RA gig as summer experience, they assume this is the case.
This second class of RA got thoroughly destroyed at my 2009 OCI. Probably the only people who had it worse off were the people with no prior legal experience who spent their 1L summers at public interest gigs. For reasons I absolutely do not understand, private employers seemed less interested in discussing public interest work, even when was clear that the work involved a good amount of legal research and writing. I split my summer between public interest and a federal judge. Every one of my 30 screening interviewers asked about the work I'd done for the judge. About four asked me about the public interest gig.