First of all, there's a common misconception amongst a lot of people that BlackBerries require BES, they do not, BlackBerries can hook directly into ActiveSync just like other smartphones without a BES.
I believe the Blackberry BIS service only supports Outlook Web Access (Not true ActiveSync), there are third party apps for ActiveSync but from my research they are a bit cumbersome to use.
Why people want to continue going down the BES route is a mystery
Well I know one reason I moved to a Blackberry and BES was that ActiveSync didn't handle notes. Additionally the iPhone didn't handle tasks. I don't know if any Andoid based devices do but none did when I was looking in January of this year.
I have a theory that it's impossible to prove anything, but I can't prove it.