Hi Thijs,
We've settled on a 3-release-per cycle to strike a good balance between the stability of the API, and access to newer features that AdWords releases on a regular basis. We also try to time the releases such that in many cases, you can skip over a release and go directly to the next one.
If you find AdWords API to change pretty frequently, i recommend giving AdWords Scripts a try. We try to shield the users as much as possible from frequent changes in Scripts, but even there, a 3 year cycle like Twitter's API is not realistic, since there are product feature changes that affect user scripts (e.g. UI discontinues a particular feature).
Cheers,
Anash P. Oommen,
AdWords API Advisor.