It is just not logical for me. TNT supports EXT_texture_env_combine and NV_texture_env_combine4. If the hw supports these functions (including cross texture references), there is no problem to support ARB_texture_env_combine and even ARB_texture_cross_bar present on GFFX. EXT_texture_env_combine and the ARB one seems to differ only in one thing - this is pointed in the Issues section, but the Errors sections says nothing about it.
Secondly, this extension can be treated as a functional superset of statndard GL env functions (MODULATE, REPLACE, BLEND, DECAL). But I cannot achieve one of these functions with RGB texture (cant remember which). The forgotten function takes as an alpha the color of the incoming fragment, while the combine function takes 1.0.