Mixes don't contain entire tracks - you only hear mixed versions. It's up to you if you try and do a lame-ass cut 'n paste and try and pass that off as a full copy of a track, but downloading mixes aren't quite the same as downloading dubs that you shouldn't have access to.
Up to the DJ whether they want to put the dubs that THEY have into one of their mixes. Bit of free advertising for the producer at any rate, so I don't think they mind toooooo much (so long as they've not asked for the dub to be kept hush-hush until more general release information is sorted, as some producers do sometimes... A little oddly, I think).