Nick Bradbury points out
an important issue with web APIs:
I created FeedDemon 1.0 in 2003, and it was the first app I wrote that relied on web APIs. Now those APIs no longer exist, and almost every version of FeedDemon since then has required massive changes due to the shifting sands of the web APIs I've relied on.
There are a tangle of issues involved here.
One is that, for sure, web APIs are not a suitable way to build a self-contained system that will at least remain internally consistent indefinitely. You couldn't build a space probe using any web APIs that you don't control. By the time it got to Jupiter the protocols might well have changed. Even barring an intentional protocol change, the service might upgrade its software and accidentally break you.
Not all software is of this character, however. Most user-facing software is expected to stay compatible with the other software a particular user is taking advantage of. Most user-facing software has a mechanism for being updated after it's deployed.
The thought experiment I use here is to consider why the Lisp Machine is no use nowadays. These systems are very impressive; think Emacs, but with a better programming language, and with multiple for-profit companies writing professional software to run on it. Nonetheless, they are useless nowadays (and Emacs is as awesome as ever). That it is so is obvious, but what is the precise reason for it?
My best answer is that the world changed around them. Even without any explicit API break, Lisp Machines just don't have integration with other forms of software that its users would find important. That whole Internet thing is a simple example.
In general, most software is only useful if it is under active maintenance. The difference between zero maintenance and a little bit of maintenance is huge. If you are considering using software isn't under maintenance, run away! If you continue to use it, you will eventually find that you have become its maintainer yourself.
Which brings me back to APIs. APIs are never perfect, and so they evolve just like any other interface. The only way this is different for web APIs is that the clients do not get to choose when to upgrade. One day, the provider updates their software and it's simply on the new API. Gilad Bracha describes this as "versionless software".
Ideally, this evolution should involve discussion between the service provider and all of the clients. Exactly how those discussions work is a rich question that is similar to any other decision process by a number of stakeholders. For an API like one offered by Google, individual clients have very little influence on the API, so you have to decide whether to take it or leave it; part of that decision involves your expectation that the service provider will treat you well. In other cases, there might be a contractual agreement between a user of the service and its provider; in that case, any API changes could be worked out as part of negotiating the contract. In still other cases, a group of software users might meet in standards committees, as happens to some degree with the HTTP protocol.
In short, I really don't think that constant interface change is a fundamental reason to avoid web APIs. Instead, it's a fundamental part of most software development that you have to keep maintaining, whether or not the APIs you program against are accessed over the web. Instead, you should be choosy about what specific APIs you depend on. Just like you wouldn't want to depend on an ancient unmaintained hunk of software, you also wouldn't want to use a hyperactively maintained hunk of web service that has a completely different API from one week to the next. Think explicitly about the API evolution story for any service you depend on, and use your judgement.