Everything can be negotiated, and non-competes are no exception. A non-compete can be more or less restrictive. Everytime I sign one I try to make sure it's reasonable. That means that you should agree not to compete in the same space you're working on. Everything broader than that you should try to negotiate.

So for any non-compete that reads "anything you do on your free time belongs to the company" you should try your best not to sign it. Negotiate! It makes sense. If you work for facebook and you're building an alternative social network on your spare time, it's reasonable for facebook to claim ownership (because you're likely using inside information). If you're developing a new music distribution software, then you should be ok.

The commitment discussion is secondary. Either you perform according to expectations or you don't. If you don't, the company can fire you. Non-competes are not tools to measure your performance. That's my understanding and the argument I'd use to negotiate non-competes. Non-competes regulate IP. Labor law regulates performance (i.e. commitment).