I mean for all we know... these class groupings might very well only be for the playtests in order to explain why the classes are appearing together in each packet and once the book is released they may very well have little to no functionality. So I dunno if its worth getting too tied up into it right now. And as far as blandness... if the only thing tying these three classes together is the Expertise ability, then we can't really say they are losing any uniqueness. They are all still as unique as they ever were, other than them sharing this one mechanic. It's not like we say that all the weapon-using classes are bland because they all share the weapon combat mechanics, or that the casters are all one big soup because they all use spell slots.
There are several feats with class group as a prereq. They seem to be serving a mechanical function.