I played a point and click adventure game that used keywords, which worked fine. There was not a lot of emotion in the dialog though; it was pretty much just a murder mystery type investigation. The main problem with keywords is when the tone of what you say doesn't match your expectations of the selection. I tried the demo for one game with keywords and wanted to say something encouraging to someone who's parents had just been killed, or something, and the actual sentence was a complete jerk response.
