I agree downloadable 3D movies would be better then retail. However us Canadians would be screwed by NetFlix as the service is not availible here. Infact I wouldn't mind seeing cartridges in major retailers not unlike the UMD. Fact is UMD had to compete against DVD, BluRay. I could get a portable DVD player for cheaper then a PSP and it would have a bigger and better screen with the DVD also having bonus material not availible on the PSP. Then theirs the sound quality which sucks on the PSP.
As someone pointed out the PSP does everything but nothing well. Fact is Nintendo could do everything fairly well without compromising the systems integrity. For example the I-Phone also does everything but people say its the best phone on the market, people say its the best MP3 player and portable movie player, then theirs even some people who prefer it to DS as the greatest portable games device (Not me but some people do).
In essense I think if Nintendo gave the feature a chance it could succeed where UMD failed. Why? well first off not to many people can afford a 3DTV so Nintendo doesn't have to worry about BluRay and TV's as competition. Then their is no other portable 3D device so no competition from the portable market, not even the mighty I-Phone can give us portable 3D movies.
It seems like a tight poll, almost as many people want the feature as not wanting it. I can't wait to see Nintendo's decision at E3!