Demand for the promo card has been high in other countries, and although it is coming a week after the movie has launched in the USA, these promo cards could still move quickly
Its obviously a much less powerful card overall, and you cant just throw it into a deck with no concern for what else is in there, but its possible there is a deck built with The Cauldron of Eternity in mind and then this deck could want Gray Merchant, but it really has to be a dedicated self-mill deck for it to work, because if you cast Gray Merchant and it dies naturally (which is the most common play pattern), it doesnt work with the Cauldron unless the Cauldron isnt on the battlefield yet
Luckily, there are plenty of useful cards in mono-red to make Spider-Punk a formidable commander
The card has diminishing returns, and its totally fine to play one, two, or three copies, if needed