It's alright to use Google's services for many purposes. However, Google Play Services is a proprietary library for Android which enables many features (such as push), but which cannot be freely installed in Android devices- the device has to adhere to some Google guidelines.

In practice, people who root their phones, run non-Google sanctioned devices such as the Kindle, etc. sideload Play Services and continue using the dependent features, but they go into a gray area.

It would be preferable not to depend on Play Services, so people don't need to do shady stuff.