Why does the Google Maps app need my phone call permission? Is there any way this is justified?
Seems like Google is becoming an evil spy to me, cause this is not the first time their apps need ridiculous permissions.
If you receive a call, an app may wish to know this. It could then free some of the phone's resources (CPU/RAM) to allow the call to be answered quickly, along with lowering or muting the navigation voice, etc. Those permissions may also be related to allowing the app to call a particular location, or even put there for a feature not compatible with your phone or fully released yet.

Individual permissions encompass access to many different things, so not every permission is explicitly used for malicious purposes. That of course doesn't mean you shouldn't be cautious.

If you really don't trust an app, use a permission manager. Your phone may have one already built in, with no root required for pre android 4.4.2 devices. Or just download one from the app store. They will allow to you disable individual permissions for specific apps. For example, most games have no business with your GPS, so disabling it won't hurt anything. Be careful, because as I stated before, some permissions are beneficial for particular purposes.