I think there are ways around this, you can always request a separate internet line in your work area, or slap a firewall between your space and their worklan, turning your workspace essentially into a lab. I mean we could probably take a look at pen testing scenarios, I'm sure you don't limit pentesters to your tools when they come into your workplace right? I wouldn't limit a high cost consultant with my tools. I think the reason companies provide a computer, is more for the benefit of the corporation, we want all code, iterations, and notes left behind, it's our product he/she worked on, how do we know all material was handed in otherwise? It's a very neat way to package all deliverables at the finish line.