I asked and you replied that you are talking about BIS. I simply added my yahoo address to BIS. There is no setting up an email client. I've had no problems on the blackberry because of it. I won't say the read/delete reconciliation has worked as advertised but that's a different matter; it has not created problems on my blackberry.
I've read of problems people have with using the google app, but simply adding the google email address to BIS as is done with other email accounts shouldn't cause any problems. If you say it does, explain, in detail.
Again, there is no setting up a client in BIS setup wizard on the device, or at your carrier's BIS web portal. What you see in options > advanced > options > default services is simply the location to set the default messaging service and default calendar. If you have added a gmail address, yahoo, windows live, whatever, you will see each of those as options.
And since 4.1 or 4.2, it's been so long I dont' remember, you have a calendar for each email address. Outlook sync's with the default calendar, and the default CICAL should be set to the same as the default CMIME in default services. This has been answered over and over and over for several years now, and I'll bet it's been answered in the last week, at least once before this. It is not a flaw or defect or problem: It is the way it is.