I develop software for a small company, and it sounds like you administer a large one... but when you say: "Dislike of portables has nothing to do with controlling you", I think you are lying. You would like to force me to install AV software (you can't, I develop in Linux), clean up my machine (whatever that means), wipe it without my permission, stop me from taking it with me, and generally have control over everything I do on the system.
I can imagine this making a certain amount of sense if computers connected to the work network had special privileges over external machines (they don't), or if we prevented remote login to our servers (we don't). Other than protecting me from my perceived incompetence, they only reason I could see for taking away control of my own machine is that you don't trust me not to run off with IP or company secrets. But I doubt you'd be able to accomplish that no matter what you do.
In short, it seems to be more about control than security. I'm not sure that up-time is an issue here either, since I can always put my laptop away and switch to the IT-administered PC on my desk if it should die.