I agree the new wording is clearer. It might be too weak; I might be wrong. I'm thinking about a hypothetical, obviously stronger version:
+2 Actions
If you have 5 differently named cards in play, +2 cards.
So this works every time, not only after the 5th play. The thing is, it just seems like once you already have 5 differently named cards in play, +2 cards, +2 actions is no longer a very strong effect generally. You probably had to draw a good bit of your deck to get to that point, as well as have bought into a diverse strategy.
Of course in the presence of Black Market, Villa, Storyteller, or Cavalry it gets a lot stronger. And in the case of Duration cards, Summon, Prince, Reserves, etc. I'm just not sure that that version would be too strong, and if it isn't, then a adding the "only after the 5th play" clause just adds a lot of words and complexity that aren't needed.