I'd probably use it myself - they were what I'd attempted to do with my version 3 a while back... but the script has fallen to the way side as I've mentioned previously I think.
1. I have quite a few instances of songs in my database which are on more than one album (especially where I have a somg on its original album, and then the same song on a compilation album) - I felt that the play count should only be updated on the album which was actually played.
When this was written, last.fm didn't track what album it was from! Nice
2. I wanted the date last played to be a bit more accurate
As you mentioned, moving to the recently played list instead was required here. Version 3 does this, but I felt it was very slow for large libraries (days!) as last.fm would take 2-3 seconds per xml file and there were thousands of requests requried. I think last.fm has sped up their servers, and your check for only getting the files since last run is something I had in mind - but I was looking for a way to cache the xml - which proved more difficult that I expected in vbscript (horrible in fact)
3. I came to this quite late. I have had an ipod for years, and have my playcounts saved up from that - I felt that scrobbles from last.fm should add to that, not replace it.
Personal preference this one I think. My phone scrobbles to last.fm, and MM scrobbles to last.fm
When I sync, the difference in *total* counts works out to the scrobbles made on my phone since last update.
If I was adding the plays from last.fm it would duplicate the plays I've done on MM.
For a simple way to release it, just upload the lastFmImport.vbs - people can replace the file in their script directories.
For a bit more work - to package it as an actual release you have to edit the MMIP - which is a glorified zip file.
use 7zip or something - you'll find 4 files - replace the vbs with yours, update the version numbers in install.ini and you're done.
Feel free to release it - just rename the package (add your username or something) perhaps so people don't get confused.