While I'd like it, I can not see it working out in theory nor in practice.

You mention security and I believe that could be overcome but it would come at a price.

Security is also a two edged sword because some security programs and depending on settings takes up a lot of resources from the computer, I'd never use norton for instance on my home computer. Then there are also some companies that makes use of key loggers which I definably am against. That might even be illegal but it has still come out that some companies makes use of those sorts of programs.

While security can be overcome, the larger issue would be support. If the machine breaks down or you need an upgrade of ram or something like that, it can be hell with every employee having a different computer.

That is one part, the larger concern I have however is the lack of knowledge and understanding from the average employee.