For some of the rationale, their announcement notes that:
The AJAX Libraries API is an attempt to make Web applications faster for developers in simple ways:
- Developers won't have to worry about getting caching setup correctly, as we will do that for you
- If another application uses the same library (much more likely), they there is a much better chance that it will be already caching on the users machine
- The network and bandwidth of the users systems will not be taxed.
One of the useful features is "a smart versioning feature that allows your application to specify a desired version with as much precision as it needs" where for example "Specifying a version of "1.8.2" will select the obvious version. This is because a fully specified version was used. Specifying a version of "1.8" would select version 1.8.4 since this is the highest versioned release in the 1.8 branch. For much the same reason, a request for "1" will end up loading version 1.9.1."
You won't hear a lot of talk about mashups built on this API but it might get a lot of under-the-covers use by Ajax developers.