Andrew,
Often a target file size, in Bytes, as well as constant and variable bit rates, bits, can be chosen when video encoding. Using a target file size will maximize the video quality for the allocated storage space available, e.g. if I wanted to save a video to a data CD then the quality would be much lower, in terms of bit rate, than if I saved it to a data DVD.
Under those circumstances a transfer speed in MB/s is more relevant than Mbps - Mbps being perhaps more relevant to an indeterminate video stream in terms of duration and hence file size? However video quality is normally best judged by the codec and bit rate, bps, used in compression as you pointed out
Edit: one can of course calculate a file size from a given bit rate, or vice versa, but that can be tricky when working with variable bit rates depending on video content.
Edited by 4M2 (Wed 09-Nov-11 17:27:42)