As I say, since Koha is gpl maybe you don't care, but maybe others might ...
And if you're going to do this, for $DEITY's sake make it an *optional* module, so that should people not want MySQL, they can just leave this module out and then they're still independent ...
In fact, i think that the "need" mySQL refers to licence constraint : if your licence disable the opportunity for the user to use something else, then mySQL must be buyed. For us, as it's gpl, the "must" is a technical constraint, which is not covered by the mySQL constraint. MANY softwares works only on mySQL (1 "replace", which is mySQL-only, and your software is mysql dependant. And almost everybody uses replace when developping on mySQL)
Yep Paul has hit the nail on the head.