The danger is that the gear and other goodies put into the cash shop will always be made far superior to what's available in the game through regular play.
If you're Blizzard and you come up with a bunch of awesome looking gear or mounts or pets, why "waste" them on regular subscribers when you can put them in the cash shop and maximize revenue.
When you play a F2P game, this sort of model makes sense. Those who never pay anything are treated like second hand citizens. However for a subscription based game you really don't want your subscribers to feel like second class citizens.
Basically, the regular subscribers could begin getting the shaft because Blizzard chooses to gate all the good stuff behind microtransactions.