It may not be the purpose of general router, but it is (or at least is becoming) standard for home routers. Not that separate devices don't work, but why have several, when one has enough resources to handle everything.
Personally I sometimes feel like ditching ROS/RB in favor of something with less limits, and basically the only reason not to is WinBox. It is by far superior to any admin interface I've ever seen and I don't want to live without it.
I understand there's no way MikroTik could fulfill everyone's crazy dreams when it comes to various programs they may want to run. MetaRouter could be a solution for that and things like usb access would help. At least until MikroTik comes with something better, i.e. some kind of custom packages.