yeah, i think a good production, engineering (getting tones) and mixing should be done by the same person for best results. there is no room for error if you want perfection, and these processes go hand in hand with each other and they really should be integrated into the same process.
Many top-tier producers *choose* not to engineer simply because the multi-tasking leaves their creative abilities/attunement impaired. Having to consider both the technical aspects of a recording, as well as the creative, isn't exactly something that goes hand-in-hand.
Consider that you're producing a record. You've spent many days now in pre-production, re-arranging songs, sorting out overdubs, harmonies etc. The band have just recorded the drums and rhythm guitars on the first track and you've noticed two things:
1) The guitar tone isn't fat enough. The recto needs some tubescream lovin', and you're considering that blending it with another amp too for the choruses may be the go.
2) There is something odd about the vocalist's delivery and note choice during the verses.
Now, being the producer, considering that your goal at the moment is to get those songs sounding just right, would you really want to go into the live space and waste hour fooling around with the guitarist, pulling tones, while the rest of the band bummed around doing nothing, or would you rather be working with them and the vocalist to try and create some 'magic' in the verse?
Can you see the benefit in having a tracking engineer there who is in tune with you and will 'know' what you mean when you say 'those guitars are sounding too small, how about we beef them up a bit, and maybe even add in another two tracks during the chorus'. He can go off and deal with the challenge of 'fattening up' the guitars, and also working out what amp he thinks will complement the recto during the choruses.
Very dumbed-down scenario, I guess, but I just wanted to outline why many in the top-tier, who can afford it, choose not to engineer and produce at the same time. You simply multi-task too much and don't bring your A-game to every aspect of the project. I frequently find myself over-extended when doing both. So what I attempt to do is work through the band's music beforehand, simply playing through some demos at home and writing down aspects that I think need tweaking. Once all that is sorted, the studio becomes a much more technical endeavor, as most of the creative aspects have been sorted out. It rarely goes off without a hitch though, so I find myself scrambling for the producer's hat mid-session, which can be hard at times, especially when you're fixated on how things are sounding from a purely aesthetic POV.