For a library, an MIT license is much more likely to get it adopted in a finished product that is accessible and maintained. A GPLed library can only get adopted in a GPLed project, which means there may be a ton of economic constraints that make the library's functionality inaccessible to anyone who doesn't have the time or expertise to build an entire application around it.
Also worth noting: to most ordinary people, GPL on a library is like a Post-It that says, "Free—take one!" on the door of a safe. It's nice to have permission, but it doesn't do much good if you don't know anyone who has the combination. The GPL-everywhere philosophy seems to assume a magical utopia where everyone is a skilled programmer fluent in multiple languages.
For finished products, though, an MIT license doesn't necessarily make sense, for the reasons lots of folks here have pointed out; namely, any big corp could compile the code unmodified, brand it, and sell it. If you want to make a complete application free and permissive for end users, GPL seems like the best way to guarantee that.