Do you actually use MIT, or something similarly permissive but without permission to redistribute in source form? Because offering a true MIT license to paying customers means they could then redistribute that version to everyone. (Of course, doing so might mean they don't get any future versions from you under that license, but still...)
For example with libogg he was primarily concerned that the world would get locked into proprietary video code that was patent encumbered. (An issue which has happened a number of times. For example at various points free software had trouble with patents on things like the compression algorithm used by the gif format, and the RSA algorithm used in encryption. Both are thankfully long expired now. But video encodings in particular have a ton of patent issues.)
I don't care if I make some schmuck billions of dollars off my work while I suffer through poverty (which won't happen from a simple DB library), I care about making the Internet more secure, even if only a little.
(My motivation was this blog post http://www.thoughtcrime.org/blog/saudi-surveillance/ by moxie)
However, for everything else, I'm inclined to use GPLv3 / AGPL and offer commercial licensing under MIT for companies (h/t 'patio11) instead.
Just my $0.02