Phyrexian Arena Phyrexian Arena provides card advantage in the most basic sense of the word: You play one card and then you either get several cards out of the deal, or force your foe to spend a card in response
The set does have a lot of bad cards in it, but it drastically altered the trajectory of Magic
As you can see, there are uses for this beyond just protecting Squall, so it will never be a dead card in any situation
The biggest downside is if Shaun gets removed, you have to exile all of your clones
Along with that, it also gives your other legendary and artifact spells flash, which is incredible
As such, I just kill an opposing Thief of Sanity and move on with my life, and it has rarely done anything bad when I play against it