unlicensed global admin reasoning

[CB1]

what impact would a global admin account that isn't licensed have on the users ability to do general admin/configuration of azure AD and other MS365 services with those privileges? 

Is there any logic why you would not assign a license to a global admin account? 

[#2]

There are only a handful of functionalities that will depend on having a license, such as eDiscovery. You definitely dont want the global admin to be using his account for "non admin" stuff though, so there is no point for him having a license. Use a separate account instead, if needed.