I kind of look at a rotator cuff tear the same as tearing an article of clothing. If you don't sew it, it will get worse with wear. So if you don't get it repaired, and you don't use it for anything excessive(talking about rotator cuff here), you might never need to fix it. But if you do tear it worse, you probably would've been better off getting it "sewn" in the first place.
I just don't see the reason to gamble on not having it get worse. With health insurance, it shouldn't be that expensive and the recovery time is fast and the rehab not that grueling.