This is what the Blogger help page says under: "Why is Blogger switching to Google Accounts".
"Google Accounts are better protected against fraud, impersonation, and abuse. In addition, by switching your Blogger account to a Google Account, it will be easier to use other Google services like AdWords, Google Groups, Google Alerts, Froogle Shopping List, Personalized Search, your Personalized Homepage, Google Answers, and many future Google services."
What I read from this is that Blogger have come up with a load of things that normal people don't actually ever use (AdWords, Froogle shopping list anyone?) and tried to put a positive spin on it, as they don't like or don't want to disclose the real answer. Surely the simple and honest explanation would be "Google is offering us loads of money, so screw you guys, we're rich!", rather than this load of crap?
I don't want to change. If I wanted to use those services, I would. But I don't. And one thing I've noticed that I don't like about Beta is that unless your email/account address is the name of your Blog alias, there seems no way to alter it (perhaps I'm wrong?).
So I'm not changing. Serves them right for not answering the question: "What if I don't want to switch?" properly in their help section.