I have 5 Evernote Premium accounts, each of which has hundreds of notes (and many folders/stacks). I find having multiple accounts helps simplify this greatly. Additionally, each account is focused on a particular aspect of my life/work, providing a degree of mental separation and focus which is important to me.
Evernote for Windows restricts to 4 the number of profiles you can easily switch between (under the 'File' menu). If you add a 5th account, you effectively get logged out of one of the other accounts. Which makes switching between accounts a painful experience.
I have tried to use my 5th account using web Evernote, but it simply doesn't provide the quality of experience of Evernote for Windows.
The limit of 4 accounts in Evernote for Windows seems arbitrary, and technically unnecessary. And from a business point of view, counterproductive: if people are happy to pay for more than 4 accounts, why would you cut off that extra revenue?
