1) No warranty for equipment if it causes it to fail
2) Cost savings is only monetary, I prefer to see everything as a whole, if I can spend $3K on a D100, I don't see why I must scrimp that $50 off a battery.
3) Nobody, NOBODY knows their own equipment as well as the original equipment manufacturer
4) Battery life & design is not consistent cross batches (that's why some say XYZ is good, some say the same XYZ battery is bad)
5) Professional batteries comes with IC chips to detect charging & calibration, 3rd party don't

Sometimes somethings cannot scrimp.