Must say that the as a general rule i agree with your current instructor - the RA hours are a total waste of time. Have to apply a bit of tough love there. We see about 1 resume a week from a young instructor and in the past have received up to 40 resume's when we place ads for staff. So culling those with very little GA time is one of the first stages. Partly because we look for people with experience flying the aircraft that we use and people that have good controlled airspace experience. It's also partly because there are next to no "standards" when it comes to RA training. Simple fact that one RA instructor could have been trained to a very high standard whilst another will not. Unless we know the school they were trained at or worked for we can't make that call. Don't get me wrong - there are some GA schools out there that do not look good on the resume either!
We're paying around $60 per flying hour for a Grade 3 GA instructor so we don't feel too bad about making harsh decisions.
If you're lucky enough to find work with a school that does both RA and GA then it might be worthwhile plugging away with the RA in the hope of moving across to GA stuff but that's probably the only scenario when it might work.