every one generally does it. i often rob loops and use them in my own productions. as long as you dont use it in the same context the original track uses and try to disguise it and make it your own, i dont see it as a problem. often if you listen to a track unmixed you'll hear a percussion loop with a high pass filter on it from another track or a bassline with the highs filtered out (this can work quite nice when reversed). obviously you dont want it as the main element in your track but more as a percussive addition. (and as long as you give the original artist a mention!!).