Google Reader thought me long ago not to depend or invest much time on Google services. Nowadays the only product that would cause me a huge problem if they decide to cancel it is Gmail.
A long-term IMAP storage is legally needed for the spooks to read your email in real-time. With POP they don't have the legal excuses, so they cannot read it. Only what's left over there.
Or maybe they'll cancel Gmail so users will transfer to (ew) Google Inbox. I sure hope not.