I think there are 2 factors for not doing water changes. 1.) You need to achieve a higher concentration for the med. to be effective. You don't want to just dump it all in there though, so that there is an acclimation period. 2.) I think some of it evaporates or comes out of the water naturally, thus you are replacing some of the lost med.
I -do not- do water changes during treatment, however, I do add additional aeration. This is very important because the med. removes o2 from the water. So, you need to lower your water level so your HOB has more 'splash', or add an air stone/sponge filter.
I have not had much luck with melafix actually treating much... BUT, when I have a fish with tattered fins, they heal up, and the fish doesn't get sick, so I guess it must be working.
I think that it's fine to use MELAFIX for what you are using it for... QTing new fish.. or when moving fish, etc... you are not using antibiotics like Laura refers too... and she is right... but with melafix, you can use it to help make things less stressful for the fish.