There is no reason to have someone else mix or master your beats.
They're just beats. I'm an engineer and don't go thru the work some of you guys put into a beat.
Reasoning: If an artist likes a beat and it's mixed halfway decent in the 1st place, they can record over the half azzed mixed beat that sounds like crap, and have their professional engineer add FX like EQ, Dynamics, Stereo Enhancers, Noise reduction, ect. When the finished track is done the beat will knock and have the same clarity as you would've got from a good mix in the 1st place. A good engineer can easily adjust freqs to make everything mesh well and sound professional, that's why it's a job everyone can't do.
If the beat is so poorly mixed that it is beyond this point(most likely due to overprocessing being done by someone with no clue of what they were doing trying to mix, ironically)the artist will request the song tracked out.
In either situation you're losing out because whether your beat is well mixed or not beforehand has little to do with the outcome of the song. keep in mind this is all assuming you're working with guys who are professionally mixing their finished product.
If not, you can give them the cleanest, best mixed beat in the world and they'll manage to f**k the quality up beyond repair. So either way, pointless.