You can't sign your rights away, and "we own all of IP you produce past, present and future," is a forsaking of rights that would naturally be exercisable by the person signing the contract.

Employment agreements are also subject to contract law concepts such as consideration[1], which would come into play when someone stops and asks "what's in it for me?" A single job as described at the beginning of the relationship through the employment agreement is probably not going to be seen as valuable enough for someone to have legitimately signed away his future productivity.

1. http://en.wikipedia.org/wiki/Consideration