I think 'use' implies consent. I have been used by dominants before in BDSM play, and I consented to it. Anal was one of my hard limits for a while, but knowing how much my Master at the time felt about it, I decided that, to make him happy, I would allow him to use my ass. It hurt the first time (he was large) but afterwards I did grow to enjoy it.
Abuse implies nonconsent. After I graduated, I moved in with a boyfriend who was 'into' the lifestyle, as he put it. Sceneing with him was all right, though I never got the kind of satisfaction from it that I got from serving my previous master. That should have been my warning flag; I should have gotten out. I didn't. One night he got drunk when he invited his friends over for a party, asked me to 'do' his rottie, and when i reminded him animals were one of my hard limits and i don't do that, he gagged me, dragged me out to his backyard, whipped me till I almost passed out (ignoring my use of the safeword and our agreed-on handsignal), then he and his friends pissed on me. Then he tied me up in the doghouse and left me there all night. A bee stung me the next morning, and i went into anaphylactic shock. (I'm allergic to bees.) His litte brother came over to let the dog out and borrow a CD, and found me out in the yard. He called an ambulance; i went to the hospital; and moved out of my now ex-boyfriend's the next week.
Abuse should never be tolerated. If the Dominant is making the submissive feel at all uncomfortable emotionally, it's time to get out. A dominant should always care for and have the welfare of the submissive in mind. If he doesn't, he doesn't deserve to have a submissive.