Not sure I'd advise to those starting out today, except for the following cases: 1. You consider enterprise or other large sales, which are not comfortable through Mac App Store. 2. You want to participate in bundles or other promotions. Technically, you don't need a store for this but you need to create a version which can be licensed outside of Mac App Store.

Personally, I started selling ImageFramer (my first Mac application) in 2006, so we first had our own store and Mac App Store came second.