Whether they last me all day or not (and the 8900 barely does), I have been charging cell phones every night while I sleep for 25 years now (the first ones had a whole 20 minutes talk time!).
It's a good routine to have, regardless - like yourself, keeps you topped off fresh in the morning, ready for the day ahead
And, PS - although I never just use the Blackberry for solely e-mail, and, nothing else, I would have to imagine the 8900 should do 3 days or more if you don't check the mails. Of course, how often you look at the mails makes the difference on battery life.