Best I can explain is that by designing them for a certain job profile, you're deciding their lives for them.
GG's point, near as I can tell, is that that's, well. Not possible, or at least not efficiently doable. It's
really hard to improve something for a specific position doing one thing and not have that skillset/capability expansion not make that thing also better at many
other things. And there still doesn't seem to be anything intrinsic to providing a genetic predisposition toward certain skillsets that's stopping the modified from doing something less than optimal. Being really good at something doesn't somehow change the firmament of reality and make it so that's the only thing you're capable of.
Basically, it's really hard to
improve something and in the process make it
less capable. You'd have to be specifically removing beneficial adaptations and... there's no
reason to
do that. It's less efficient, notably so. You get more use out of a smart critter with legs, with less infrastructure investment, than you do from ones without. There's no financial (or any other, really -- any control issues can be handled on the social side much more effectively.) incentive to breed a less effective critter, y'know?
Though I guess that's tangential to the slave thing. I'm not really sure about that one, either. It takes more than just "better at deskwork" to turn something into a slave, I'd say, but perhaps TMS can clarify the point.