I only half agree. There is a much different dynamic playing against someone if you see their card, and if you don't. You have to consider what cards they might have or what the more common cards are when you can't see them, which is, itself, an important skill of competitive Triple Triad. Against NPCs, All Open is very welcome (espcially if Plus is in effect), but that's a different matter.

Honestly, I'd take Three Open as a non-rule default, with "All Open" and "All Hidden" being two separate rules of their own.