Presonus Eris E3.5 BT Active Studio Monitors (Set of 2)
The answer is, in my opinion, that they're more than capable of fulfilling the role as keyboard monitors for practice, and do an excellent job for the price. For anything more than practicing, you'd obviously need a lot more power. I can't compare to dedicated keyboard monitors such as the Nords - I'm sure the difference would be notable - but these are doing me fine. Maybe one day I'll add a sub to get more authentic piano vibrations. Or just get a real piano...
I also own Eris 5's for studio work and the 3.5's definitely feel like a lower quality product (a lot lighter and less sleek in design), but at 35% of the price corners obviously have to be cut somewhere, and this isn't an issue for me.