I'm with you. One of the reasons i bought a C-14 is the easily removable bags.
There are a number of reasons one might want to use bag liners:
1) The luggage ("bags") are heavy
2) The luggage could get scratched or damaged during removal/installation/transport
3) If you have retainer clips or bars, it will take additional time to remove/install those
4) Having bag liners keeps loose stuff organized and upright
5) Bag liners can also make better use of the available space, since it is difficult to pack the luggage fully due to the half-open state (and trying to stop pinching, etc)
6) It *is* faster to open the luggage and grab a bag, and return it later. Not tremendously faster, though.
7) Less wear on the retaining mechanisms (not that this is a big deal).
I can see a case (no pun intended) for taking the luggage on/off AND for using bag liners. Both can be very useful. I have done both and it just depends on the situation.