It's too hard to get the full "6 degrees of freedom" control with mouse and keyboard. Especially considering you only have two axes with non-binary input. But even ignoring that, you can get pitch and roll (or yaw) on your mouse, but then you need throttle up, throttle back, yaw left, yaw right, strafe left, strafe right, strafe up, and strafe down on your keyboard, which is an annoyingly large set of keys to keep track of, without even taking any other functions of your ship into account. And it leaves you trying to hit 3-4 keys at once quite commonly, which your keyboard is likely to not support.
My x52 gives me access to all those things at once, so I can use them in any combination. Granted, my strafe controls are binary, but you can't have everything. A little thumbstick for strafe would be neat, but the hat under my index finger on the throttle works fine. And I can be doing a left roll, a right yaw, a pitch up, and strafe left and up all at once without my computer beeping because I was hitting too many keys at once and freaked out my keyboard's pcb.