I'm not sure that it's actually a deal-breaker. You can mark your notebooks public or private. A public notebook might be useful to share stuff with the world. A private notebook seems to imply that it is private and won't be aggregated. I may check their forums to see what they say.
As with any SaaS type app, you may still want to be careful, but it doesn't seem like they are out to aggregate anything in a non-public notebook.
See also this post on their blog:
http://blog.evernote.com/2008/04/15/...-and-security/
I realize it's not a perfect answer, but it gives me a little more of a sense of security about the whole thing.