That's a plus for me because we usually do two weeks. We could definitely spend 3 days at HII on the front or back end.
To me then it becomes "given all the caveats (can you afford it, are you going at a time you need the home resort window, etc) - great idea." It is a lovely resort. HHI is a great place to spend some time (I prefer the off season, too many people in the Summer), WDW for twelve days is ALOT of WDW - and shortening that by three days gives you plenty of time to see everything you want to see at a slow pace, and still spend a few days somewhere else were people are moving at a slower pace (again, I like the HHI off season - in the on season, the pace isn't that much slower than WDW).