> Innovate, don't duplicate
> Don’t make a substitute for Google Photos or use this API to create, train, or improve (directly or indirectly) a similar or competing product or service. For instance, if your app’s primary aim is to provide a general purpose photo gallery app, it’s a substitute for Google Photos.
IANAL, but this seems to rule out the use of this API in a number of places where it would be useful (plugin for open source photo management seems to be directly targeted etc).
I can understand why they don't want to allow me to train my image recognition software using their API, but this phrasing makes me really wonder what it is for except syncing.
Also, generally the wording is vague enough to cover almost anything, which means at some point, sooner or later you might have to rely on the judgement of some random future googler to prevent all your integrations being destroyed.