Actually the toolbar covers everything the memory firewall does. Not sure why they choose to implement the toolbar and not the full memory firewall...
They did a toolbar, so that CMF tech can be inside the browser to protect you better from "drive by" websites. It does protect your whole system like CMF, but from what I understand, it doesn't have 100% of the CMF functionality yet. Like Josh said, it will in the future.

It's very complicated technology and they needed to be inside the browser space to protect better, so they made the toolbar and while they were at it, they made the toolbar from Ask.com so they can get some revenue from it.

Installing and using it is the best way to support Comodo and their efforts for providing free software. They made the toolbar as un-obtrusive as possible and once you install it, you can even uninstall it, so it's an anti-toolbar since you can't think of it in the traditional evil forced upon you secretly installed toolbar.

Cheers.