As i've received an explanation via PM i figured i will post it here for someone who was confused as myself. Old AIP client (which integrates with Office apps) was controlled via Azure portal and wasn't compatible with unified labels available in Security & Compliance. New AIP client will work with labels in SCC (presumably this administration bit will move to new Compliance center at some point) and should cover all Information Protection products/usage cases. New one is not covering all features of old one yet, but MS plans to reach 90% parity by 2019 end. Old one won't receive new features from now on.
I just wonder, when and whether this client will become an integral part of Office 365 package, so you won't have to install it additionally. Compliance seems like a must have thing nowadays, so if you install Office for all employees, all should have AIP capabilities out of the box. It's not that you only install AIP client just for a few users. This won't help with information protection, so i wish it would be shipped along with Office (less deployments to manage).