Starting this design task because the possibility of a remove doubles modifier was suggested (in replies to D443).
The current design of the modifier stack creates an entire new mesh for each modifier, this means its not well suited to having small (fine-grained) editing operations as modifiers.
Which is why we don't already have many basic operations as modifiers.
So for this design task I like to gather some good use cases where a remove-doubles modifier would be used, to consider if its worth adding.
- Merging could use a boundary only option, since in many cases you want to zip up two boundaries (will be faster too).
- Merging 2x solid edges (result of solidify modifier for example), will create internal faces, we could have an option to remove duplicate faces (which are likely to be internal).
- ... Further, merging overlapping faces isn't likely to give useful results (it can pick first face but not make a useful choice).