I agree with you 100%....
With 10.2 I wanted to switch to Mail.app, but after only a week of using it, I couldn't settle in like I have with other applications.
I am so used to Entourage having everything included in one program. I like iCal, and Address Book, but why mess with opening multiple applications that really should be linked. In Entourage I can get an email from a co-worker requesting a meeting with me and I can just flip to my weekly calendar and see what's open.
I want to use iCal, Mail and the Address book, I just can't see a reason to 'switch' to them.