Also, it takes away the whole, "I've seen a Banksy" aspect. The fleeting nature is one of the aspects that makes it great, like a meteor shower or a "murmuration". I also like that street art evolves as the surrounding evolve. I can see both sides too but think I more "anti"-gallery.
the dynamic here is a little nuanced, however. there is a big struggle in detroit between preservationists and those who want to tear everything down and erase the memory of the failed past. most of the younger crowd--those who are repopulating the city, including the owners of 555 Gallery who now possess the Banksy--fall on the side of preservation; most of the city's establishment fall on the side of "progress" (for lack of a better word). This was best embodied by a big fight over Tiger Stadium, which was involved in a court battle for years, but the site now sits as a fenced in vacant lot. The Banksy was moved, I think, so that in the future, when all of the court battle are lost and the whole city is a vacant lot, there will be a reminder that there used to be something cool here. Or, the cynical view is that they saw a piece by a famous artist that was free for the taking, and they wanted to increase traffic to their gallery. That can't be discounted.