I ran into this issue where I wanted to buy Affinity Photo on sale after winning an Apple Design Award while I still only had Snow Leopard systems. There was absolutely no way to buy the application on promotion on any of my four Macs.
In the end, I had to create an account on a colleague's work computer with Mountain Lion to make the purchase. I was then able to successfully download and install Affinity Photo on my girlfriend's MacBook Pro (she can use it too) and on a Mac Pro I bought later for multimedia work and installed Yosemite on.
On the other hand, what does work is moving large installs around from account to account via sneakerware. While the applications do need to be in the application folder, for the most part you can just copy your application bundle in. I was able to do this with the huge 20 GB Apple store version of Pathways to Eternity (not that I'll have ever have time or patience to play it, as I obviously enjoy writing detailed tech answers on apple.stackexchange.com more than playing games). This workaround saves an enormous amount of time and bandwidth for large applications.
Even more disturbing is the situation with OS upgrades. If you have not downloaded an OS upgrade before the next one comes around, you will never be able to gain access to that version.
More precisely, in my situation with Snow Leopard, I never downloaded Lion. I cannot download Lion now: I'm happy about that, 10.7 was a dud. No harm done. I did download Mountain Lion (10.8). I can download Mountain Lion. Good. On the other hand I didn't download Mavericks (10.8) before Yosemite (10.10) came out. I'll never be able to download Mavericks from my Apple account. The same issue may come up with El Capitan 10.11 (although I don't think I want El Capitan any more than I wanted Lion).
Workaround: once you've downloaded an OS upgrade you can save it and burn it to USB key for use on multiple computers and even other people's computers. I recommend doing this. This is what I would have to do to get Mavericks.
