Mode7ex could create DOOM if DOOM didn't have walls, or if the walls had no texture (kinda like an inverse Wolfenstein!). DOOM uses it's own rendering engine to create the illusion of 3D.

Mode7ex also can cause a pretty major performance hit and requiring a PC powerful enough to play DOOM 3 for a game with the visual appearance of DOOM 1 is hardly ideal...

I usually find myself using Mode7ex just for background effects and the like. It makes good arena floors in fighting games and small, repeating textures with height mapping for the ground can be great in Starfox-like games.