Great question! I find that adopting GTD has made me much more aware when my "defining work" activities exceed my "doing work" ones, which then gives me *choice* - if the ratio is off, I adjust. Too few new inputs? Then I need to "stir the pot" a bit. Too many? Then renegotiate, using my Someday/Maybe file. One lesson I've learned is that the level of commitment I'm able to handle is dynamic, and GTD allows me to better stay in step with it!
For those interested, I wrote a little about it here: When inputs exceed your workflow system's capacity.