For the convenience and security sake I buy only at AppStore , there is no need to send registration somewhere else! It is the same as you would buy your groceries only at producer and completely avoided supermarket!
Your analogy is a little busted. The App Store is a lot more like a supermarket, and "buy your groceries only at producer" is the pretty much equivalent to buying software directly from the developer (a farmer's market, I guess?)
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My reason for preferring the App Store is licensing, plain and simple. Too many app developers have policies more restrictive than the store does, which is incredibly irksome when you have multiple Macs.
Really depends on the developer. I buy OmniFocus for Mac directly from Omni Group. It's $80 that goes 100% to them instead of 30% going to Apple for no reason. Omni doesn't care at all that I'm installing it on two Macs that I own. Also I was able to install it on a work Mac they gave me that I wasn't about to sign into with my Apple ID.
Even Final Draft is fairly permissive these days -- I have it installed on both my Macs, and can pretty readily move registration to wherever I want (max 2 machines I believe), deauthorizing one if I need to. Again, I wouldn't need to tie a whole computer to my Apple ID just to install it, AND I could give the license away to a friend if I wanted to, like in the olden days of buying a CD, or a book, or whatever other piece of media. I like that. And although, like most screenwriters I have no huge love for the FD people, they may as well get all of my money instead of 70% of it.
I also use Audio Hijack Pro a lot. That isn't even ON the App Store because it can't function with the system restrictions imposed there. Got that installed on two Macs as well. Like OmniFocus above, it's just my name and a registration code. Nothing restrictive there.
I know there have been some pretty restrictive copy protections in the past, but frankly, portability of license and paying the developer fully and directly trumps the convenience of the App Store, in my opinion.