Why don't I just use the 1.2 spec? I'll tell you why; non extended OpenGL's blending plain old stinks. Only four functions are available (REPLACE, MODULATE, DECAL, and BLEND). Sure, you can do alot with the 1.2 spec, but alot isn't enough in this case.
The widely supported EXT_texture_env_combine extension helps alot, but still can't match the N64's blending. No crossbarring, one constant color per stage, no subtraction, etc... I could do multipass...but that gets slow, sloppy, and you run into problems when you want to do stuff with alpha.
That's why I'm using NV_register_combiners to do the blending. With it I can perfectly emulate almost any combine mode the N64 can do.
Radeon cards can do good blending too, but they use a completely different set of extensions, so I have to write another combiner specifically for them, and since I don't have a Radeon card to test with...
