I will never, ever buy an iOS or Mac app on a subscription basis. No exceptions. Apps must be purchased at a fair, one-time price, or be free, or I won't use them. I expect to have a perpetual license to use my purchased version and get minor updates at no cost, and then I can choose to pay for major upgrades if I need the new feature set and/or compatibility those bring to the table. The app developers will continue to receive funding for ongoing support and development over time, to the extent their major updates remain relevant and compelling.
However, with iOS apps and Mac apps purchased from the App Store, that means developers must offer separate "upgrade" apps over time, which is inelegant and justifiably unpopular with many users. I believe that's still far better than a subscription model, though. With a subscription model, developers are paid for users' sustained app usage regardless of whether their app updates prove to be relevant and compelling. Then, of course, there's the basic issue of having to endure yet another recurring payment obligation... and another... and another....
For institutions using vertical market applications and/or needing comprehensive support options, I can see the subscription model being [possibly] appropriate. But for most individuals I don't think it makes sense at all.
On a similar note, I will never, ever rent my music on a subscription basis.
The subscription thing is one of my most hated Apple strategies ever. I'd much rather have the ability to purchase "app upgrades" for major new versions from the iOS and Mac App Stores (as opposed to having developers release entirely "new" apps to recoup their development and support costs). Free trials from the App Stores would be good too (but maybe that feature already exists?).
