What's wrong with using Google Play Services?

Correct me if I'm wrong, but I assume it has to do with message notifications. So, by using GCM, Signal would be leaking some metadata about when and who, etc. I assume. But wouldn't someone be able to get that same information from your ISP (with a little more work)?

You're losing the benefits of longer battery life for basically nothing.

Security isn't absolute. I don't know why this blogger has the attitude that there is such a thing.