I think it also depends on how strong the other areas of your application are. If you are 4.0 and 1600, you could probably get in most places with the bare minimum "vet experience" - I think most schools require at least like 100 or something, so you're obviously well over that. If you are more average or low-end as far as the academics go, than yes, you should try to diversify your experience a little, as that will help to balance out the weak numbers and make your app a little more attractive.
You didn't mention if you are working FT or PT, but either way over 3.5 years you've probably got plenty of SA experience. If you have the time (ie, won't be blowing off classes and hurting your grades/GRE, etc.) and can hook up with some good vets in other areas, I would say yeah, get some more diverse experience, it can't hurt and will just give your app a little more flavor.
FYI, I got accepted to Kansas with zero experience in LA, food, equine - and KSU is obviously big in those areas. I had nothing but SA experience, but in several different settings. But, I did get rejected from my other 5 schools, and who knows, maybe if I had a few hours in those other areas it might have made a difference.
