...not because it's just a bad idea to provide cutting edge equipment to do the job. It's a bad idea because of one thing...legal liability.
Right now, companies all over the world, are battling governments, civil rights unions, employee unions, activist organizations and so one over the idea of personal privacy. Personal privacy doesn't really exist but we like to make up the illusion that it does by saying something is mine and you can't have it or tell me what to do with it. It's mine, mine, mine, all mine, keep your grubby hands off it you evil, faceless corporation!
That's all well and good until it comes time to clean up a mess like a data spill or a hostile attack on a system. See, corporations have a much easier time enforcing computing policies when they provide the equipment, network and other computing equipment for their employees. When they own the equipment, there is no longer a question of "civil rights" because of the idea of private property. Just like you, at home, reserve the right to limit public access to your home and all the things you have in and around it in any way you see fit, so do the corporations. Democracy stops at the front door in the interests of the more bureaucratic but often more efficient hierarchy of a private, tiered dictatorship.
When the company owns the equipment, if they allow you any level of personal use or personal privacy beyond the minimal amounts that most labor laws require, it's by courtesy only. They can tell you what you can and can't do with their private equipment. That extends to whatever security, anti-virus, anti-malware and proxy level they choose to instantiate in their systems to protect company assets and property. Sure you can lobby against it and whine like a petulant child but in reality, you don't have much of a foot to stand on.
If you allow workers to use their own machines, you open a gigantic security hole as well a massive logistical problem in maintaining and securing your networks and shared resources. How do you ensure that users are keeping their systems up to date with patches and updates? How do you ensure they are using a compatible version of an OS? How do you even ensure they are using a LEGAL copy and not a pirated version rife with back doors and other little nasties? What do you do about limiting network access? You could use a VPN system with something like RSA's SecureID system but then you are talking massive amounts of system overhead with poor network performance.
There is a host of problems associated with the idea that I could list for hours. Those are all technical. They do not even address the human factor. Even as it is now, when one employee gets a system upgrade while another languishes away in obsolete-system-land, it starts petulant in-fighting and envious behavior until the other employees are satiated. That only lasts until the next round of upgrades. What happens when Joe is still stuck with, say, a Dell C600 'cause that's all he can afford after paying Little Joey's college tuition and Ned comes in with a brand new MacBook Air? The jealousy will still be there. It will probably foster dissent about Ned's level of compensation vs. his perceived contribution as well. That bring a whole new mess of problems for HR. You're no longer managing people as much as you are babysitting them.
Maybe there is a bottom line benefit to the idea. However, people have an amazing affect on a bottom line in ways that most management seems to have an inability to comprehend. I'll leave it all at that because I could easily go on for pages about this. Especially since I'm one of those system security weenies that would have to deal with the aftermath of implementing such an idea. The words nuclear holocaust come to mind to describe what the networks would look like afterwords.