GLSL compiler is build in driver, while fxPascal translates to ARB vertex and pixel shader assmbler. And this really makes a reason!

For example:
* GLSL is more feature rich.
* Vendor driver can better optimize your code at higher level (and I bet vendor drivers doen't know anything about "style" & shortcuts of assembler code produced by fxPascal).

Sure, if anyone aims at GeForce3 level hardware - when it's better to code directly in assembler.