I appreciate this for the energy it adds to gameplay. It's not "realistic", perhaps, but it is a wonderful mechanic for players that like to "live with the consequences". In real life, I can remember numerous times where I have been in a position to make an important decision. It feels like I could debate forever about it, and even if I had days to decide, the pressure was on. The other person needed an answer -- they weren't going to wait forever. Whether it was business, romance, an intense argument, etc...it was time to jump. A countdown timer is a fun way of simulating that energy for a game.Timed dialogues are what kept me from buying Alpha Protocol, because it's not realist (IRL you don't have to use an interface which makes you choose from a limited number of options) nor fun.
At least make sur it is an easyly moddable thing, as I don't want to pass of the game just because it wants to scream "ACTION !" instead of lettinf you actually roleplay things.
But I agree that including such things as an option is the way to go. Let the player decide.