Our company, like many others, tries to block all web-based email, which really diminishes the value of Gmail and the other biggies. While the company's worthy objective is to eliminate virus entry points, webmail blocking is definitely an inconvenience to employees who are trying to balance their home and work life.
For example, my Gmail account loses a lot of its value when I can only get to it at certain times, so it gets used less and less. I do check it, but not as often as I would otherwise. I end up using my company email for most personal communication, even if it all ends up as company property. Not an ideal solution, but it is the most workable for me.
The key word in the first paragraph, though, is "tries." The company can't block all webmail, because it doesn't know all webmail providers. Is taking out the biggies good enough? It probably is. But when people try to route around IT, you may end up with bigger problems than the ones you were originally addressing.