>"For pro apps with dedicated audiences (see: Adobe, Ableton, Avid, Nuance) which is just about anything with a plugin architecture, the App Store's distribution is unnecessary."
It is true that inn terms of dedicated (aka existing) audiences or customers, access to is probably still unnecessary, however it is clearly a disadvantage when it comes to generating new users or providing upgrades to existing users.
The economics for pro apps are completely different than anything an app store customer can reference. A lot of the market leaders (Pro Tools, Adobe Suite, Avid) have succeeded despite being orders of magnitude more expensive than competitors and erecting barriers for the average user accessing them (byzantine copyright protection, proprietary hardware requirements). There are always lots of cheap and even reputable alternatives, but the leaders usually capitalize by being early to market and basically owning the leaders in the field. Once you have a community of the best and brightest using/evangelizing your software, distribution is a pretty secondary concern.
It is true that inn terms of dedicated (aka existing) audiences or customers, access to is probably still unnecessary, however it is clearly a disadvantage when it comes to generating new users or providing upgrades to existing users.