JSoftware
Well, if you are enabled lighting, and do not need strict behaviour / precision of "standart" lighting -> when you are somethat right.

But from the other side: fixed function T&L setup on modern chips translated to vertex shader, but it's often more optimized than it's possible by using standart vertex shader assembler syntax. You can verify it by youself looking at results of D3DRightmark - often used by www.digit-life.com, www.beyond3d.com and some other sites.