It's just commonly known at firms that a lot of schools create extra RA gigs if they have extra students without jobs.
Yeah, that's definitely true, but firms will be able to tell from your GPA whether you are one of those people they have to create a job for or whether you are someone who could have done something else but chose to be an RA. (And yes, we are out there!) I get that having an RA gig is often a signal that you were unable to get another job. But if it's clear from everything else (you have a high GPA, you interview well, etc.) that you are not one of those people who got stuck being an RA, then the reason for not doing it doesn't apply to you, and you shouldn't dismiss it right away if you are really excited about it and you can get an RA-ship with a well-known prof.