Personally, I actually prefer the replacement technique, and that may make me quite the minority here. Usually, I'll only have to make edits in the 5 minute window if I made a mistake somewhere. And there are two cases where I'd like the summary of the past to be cleared - if my fix of the mistake invalidates the content of the original summary, or if the mistake is in the summary.
This is especially the case when secondary and tertiary edits cancel out the effects of the original edit. If I tagged something using my edit powers, then did some typo corrections, I would like that if I changed what tag I added, then as far as the revisions list is concerned I never added the first tag at all. If my original edit summary indicated this (and I usually do comment about tagging), then I'd like to be able to remove that as well so that there are no traces of the act. This is one of the secondary reasons for the 5 minute window to exist in the first place - to allow the editor to backpedal any minor and/or silly errors without the system recording it.
In the second situation, this is the same. If I made a typo in my edit summary, I don't mind taking the extra steps necessary (two edits minimum if no new edit content is necessary) in order to fix up my edit summary to look as if I got it right in the first place.
However, I don't mind refining my edit summaries with later input, and also don't mind just plain retyping my edit summary when I don't actually need to change it in secondary and later edits. This probably weighs in heavily with why I prefer to keep the replacement technique.