Another vote for this feature. Should be able to turn sync on and off on a per-notebook basis so that devices with limited capabilities or roles do not need to have an account's entire note database stored locally.
Example: Want to have one notebook on a client-furnished computer, must not update notebooks related to other clients. Forcing sync of everything would violate all sorts of contractual agreements.
Example: Don't have enough storage on a small tablet/laptop/phone. Self-explanatory. It's great to be able to compose and edit a few notes on a phone, but it's preposterous to pay a many-gigabyte price for doing so.
Example: Don't want to place personal notebooks on a business computer. Requiring this forces people to break company policies, potentially break laws/regulations, etc.
Example: Don't want to have "archived" (i.e., obsolete and no longer used, but kept for record-keeping purposes) notebooks show up on every new device. Big reason we have the cloud, y'know, is so we don't have to to keep copies of everything everywhere.
The suggestion of using separate accounts leads to a proliferation of accounts and authentication data, which is never a good thing. It is also an abuse of Evernote's free accounts, and an unacceptable cost if non-free accounts are required. And, of course, there is the ever-present risk that Eevrnote will change the terms of those free accounts.