I would vote for this as well either as a user choice and/or as additional distance information. A couple of reasons:
1) As gmngueko suggested, if the GPS is either not available such as poor reception or for those that do very long distance/time workout and are concerned about battery life.
2) For those that are like me and workout indoors a few times a week due to regimen, weather, time, mood, etc. the GPS is obviously not useful. I used the HxM while on a low impact elliptical machine recently and I got heart rate, time, cadence, calories so it would be nice also to record the distance.
Easy for me to say as I don't have to do the programming work!
As you can see there is already quite some people with the HxM :-) so hopefully you will implement it.
But a first step could be to dump in the csv file the accelerometer info already (so one could analyse a play around with it).
do you use the distance speed calculate using the HxM information ?
my idea would be to have it as a backup of the GPS speed distance when the GPS signal is not usable.