I got a Bike Friday NWT, partly thinking I would commute on it, and I never have. There are a few reasons. The normal bike is already sitting there ready to go - no unfolding required. I paid $250 for it used, so it would be much less of a loss if stolen, compare to $2000 for the BF. In summer I arrive at work on the verge of sweating excessively, and I stand outside by the racks and cool off for a couple of minutes. If I had to fold the BF and carry it up to my office, that would be just enough extra effort to make me drip (a rollable Tikit would help here). Some of the downtown streets near the office are badly rutted and potholed, and larger wheels give a smoother ride.
If the folder was my only bike, I might commute on it, but since I have the handier alternative right by my door, I don't.