'counts as' is the single greatest thing that GW has ever done.
I fully agree. Since GW has a thing with not updating models and some of these models have gone completely unchanged for 3-4 editions, counts-as is the single best way to give them a face lift and to give your own army some uniqueness and personality.
Ontop of the fact that you can "counts as" just about anything as long as it is modeled correctly..like Salamander Bikers on Cold Ones...Space Wolf Bikers on..well..Wolves. Imperial Guardsmen on those 2 wheel upright thingies as Sentinels..it all depends on them being modeled correctly as far as gear goes. And "counts as" can also be good for doubling a models usage.
My Librarian I made counts as Mephiston in larger games..and in smaller games, just an Epistolary. It cuts down on the confusion of using a Mephiston model as a normal librarian, as well as prevents people from going "SHIT! MEPH! FOCUS FIRE!" and vaping him in 1 round..when he's not being used as Meph.
So, yeah..counts-as is amazing. IG 'dex for Traitor Guard....in the immortal words of Rob Schneider..."YOU CAN DO IT! BITE HIS FREAKING HEAD OFF!"