Developing elegant, light, fast apps people can actually use is not easy: OK, understood, that is why there is a lot of garbage out there. It is also why a product like Evernote has what appears to be a cracker jack team of excellent coders.
Developing the ability to have multiple accounts sign in and see the same content, not that difficult and is done all the time, while also allowing apps to stay light and zippy.
Would someone be more likely to buy and use Evernote because of this feature?: Absolutely! It is the one feature that keeps me from moving into Evernote fully to begin with (and apparently everyone else in this thread who wants the feature).
Would it cut down on Evernote's revenue: Probably initially, since to have a paid account it is tied to 1 unique email, preventing teams from gaming the system and sharing one Evernote account. However, I think it would adjust and revenues would increase due to the people who could fully move in and adopt the model completely.
The real question is not CAN it be done or SHOULD it be done (both I think are yes from all the posts), but WOULD it be beneficial to Evernote to do it? My answer is yes: the people who are trying to get something for free and game the system, likely don't add up to all the people out there with multiple emails that they use daily.
Either way: Evernonte is a great product that could be even better with this feature.