For the rest of this turn after this card resolves, you cannot Special Summon, except Zombie monsters
While this does not mean you should include crappy cards in your deck, it does mean every now and then having an under-used strategy or combo available to you as a trump card is an important part of building your deck
Gray Merchant is a card that serves many purposes in a deck it can kill your opponent without ever needing an attack phase (especially in multiples) and it can gain enough life to give you time to stabilize
Evaluating the entire package is much easier
It gave us tons of legendary creatures for Commander (though after the fact), and it laid a lot of the foundations for what would later become the original Modern Masters Draft format, a format that many would hail as nifty. How naive we were to think casual-alienating in a bubble mechanics were toxic then (they are, for business purposes, but that really hasnt stopped it from happening)
All proxies are clearly marked as such, using alternate art