i think they can be good even for the sake of feeling more comfortable and confident. in BJJ, everybody has their own personal injury/weak joint. i think it's nice just to use a sleeve or something for the while until you aren't as afraid from any freak accident from happening. but then again, freak accidents generally should not occur if you're just working out (ideally).
several guys there have used elbow sleeves. although it can help keep stuff warm, i sometimes get annoyed when it has made inflamed stuff feel more inflamed just by heating up so much. if you're just working out though, i don't think it would get too hot. you generally dont' get nearly as hot and sweaty as you do when you're rollin with another dude (especially if you're wearing a gi).