Generally speaking, almost ZERO producers mix their own records. It's slightly more common for a rock producer to mix their own record as opposed to a hip-hop producer.
I have an argument to make: First off, most of you guys do have good points about producers mixing and not mixing their material. But, ZERO producers mixing their own records??? Maybe in Hip Hop. But certainly true, that in Rock music some, if not most producers mix their own stuff. But in Dance & House music, almost every producer/remixer (who is a DJ), MIX their own stuff. I'm one of those. How the hell am I supposed to produce or remix a track and hand it off to someone, and that particular mix engineer is supposed to know how I want my track to sound, as far as effects, panning, compression, filtering, WHEN to add automation to the filter cutoff and/or resonance, and to what parts, FADE-INS/OUTS, when to reverse a crash cymbal or a kick for effects purposes, how or when to add a splashing reverb effect (with or without a following LFO to (let's say) one particular clap (not the entire clap line), If or when to add a lowpass filter to a kick on the break and add a long decayed reverb to it, etc??? As a DJ, I'm constantly playing music (especially through different sound systems), I know what a finished track is supposed to sound like. And that knowledge also, has come from years of DJ'ing and producing and mixing hands-on, and being in studios (especially in the analog days when almost every single studio had a 2 inch (24-track) tape recorder) HEARING and watching what is being done. There's just so much more, that I can keep going. Even if I tell them (the mix engineer) in detail, granted, yes, it may sound good, but bottom line, it will still never sound exactly how I want it to sound. They are going mix it there way no matter what. Now, if a producer has no idea how to mix, then OBVIOUSLY, that changes everything. AND there are a lot of producers out there who claim they know how to mix, and they don't. Again, in Hip Hop, this might be 100% true and best off. But in Dance music, it is a whole nother ball game. Now when it comes to mastering, that is a completely other ball game. I do not and will never try to master my own tracks. Maybe I'll just add very subtle compression on the master channel with a plugin like the Waves SSL Comp or the Vintage Warmer, but absolutely nothing else. I know this might not be of your taste in music (generally speaking) nor Hip Hop, but as an example, if you follow this link:
http://soundcloud.com/djspinn/dj-spinn-i-just-go-2010-new, I have a Progressive Dance track on there, that I produced from scratch for one of my artists and mixed it completely MYSELF. And again, the point I'm trying to make is that if you know what you're doing, then you can do it.