Interesting. For $10USD/month, you get unlimited access to the latest versions of what is promised to be around 300 commercial Mac apps. Looking at the selection so far (about 50 apps), these appear to be of the sort that usually appear in popular app bundles (e.g. StackSocial etc), in which you can buy apps outright for a tiny fraction of the list price (quite often at a 99% reduction). I have a few of these already, for which I paid an average of 1 or 2 dollars apiece, albeit that they came with a bunch of useless junk that I did not need or already owned, so perhaps it's more realistic to say they average more like $10 apiece. Either way, they can already be purchased for very little money, if you have the patience to wait for the right bundle to arrive. So why bother with this?

The main advantage of SetApp's model is that, unlike those in bundles, which often nag you to upgrade to the next version at a far higher price than you paid almost as soon as you get them, you always get the latest version. It is also nice to have on-demand access to a whole library at any time: if you can wait for a few months they will probably turn up in a cheap pay-what-you-want app bundle anyway, but they are only rarely available when you actually need them.  I guess there is a small advantage in the curation service, but there are plenty of much better and less inherently biased ways to discover tools that are worth having. 

The very notable disadvantage is that you never actually own the apps - once you stop subscribing or the company changes conditions/goes bust, you lose access to them. For ephemerally useful things like disk utilities, conversion tools, etc this is no great hassle but, for things that save files in proprietary formats or supply a cloud service (many of them) this would be a massive pain. As there is (presumably) some mechanism for updating and checking licences, this might also be an even more massive pain if you happen to be on a plane or out of network range when either the app checks in or the licence is renewed. I don't know which method SetApp uses to ensure that you have a subscription but, one way or another, lack of network access at some point in the proceedings could really screw things up. When (with high probability) SetApp goes bust, you will be left high and dry. Also, I'm guessing that it is unlikely that I would want more than a dozen or thereabouts of these in any given year, so each would cost me about $10 every year at the best of times. Though that might be acceptable for a major bit of software on which one's livelihood depends, for the kind of software that is currently on show, that's quite a lot of money, notwithstanding the convenience of being able to pick up a specialist tool when you need it at no extra cost. 

This is a fairly extreme assault on software ownership but closed-source software of all varieties suffers from the same basic problem: you don't own the software that you buy.  Unlike use-once objects like movies or books, software tends to be of continuing value. The obvious solution is to avoid closed-source altogether and go for open source right the way down the stack: that's always my preference. Unfortunately, there are still commercial apps that I find useful enough to pay for and, unfortunately, software decays. Even if you buy something outright that does the job perfectly, at some point the surrounding ecosystems (the operating system, network, net services, etc) will most likely render it useless or positively dangerous at some point. There are also some doubly annoying cases where companies stop supporting versions, lose databases, or get taken over by other companies, so software that you once owned and paid for is suddenly no longer yours (Cyberduck, I'm looking at you). Worst of all are those that depend on a cloud service over which you have no control at all and that will almost definitely go bust, or get taken over, or be subject to cyberattack, or government privacy breaches, or be unavailable when you need it, or that will change terms and conditions at some point to your extreme disadantage. Though there may be a small niche for such things and the immediate costs are often low enough to be tempting, as a mainstream approach to software provision, it is totally unsustainable.