If Yahenni and a creature an opponent controls die simultaneously (perhaps because they fought or were in combat together), Yahenni won't be on the battlefield as its triggered ability resolves
(It didn't become a keyword until Innistrad many years later.) We then became enamored by the idea that Tahngarth could fight while he was attacking
Strategic complexity This is not only fine at common (provided it doesn't have one of the other two types of complexity) but encouraged, as it helps to create depth for the more advanced players
One of the best decks in the recent history of Modern is Domain Zoo, which uses Wild Nacatl for this exact purpose
From the announcement trailer, Pokmon TCG Pocket seems to have been built in the same mold, a free-to-start game where you collect cards and challenge other players to quick and breezy battles
Of course, it's very possible that the next batch of battles are just sieges again, but the designers left themselves open to possible variations