I can only speak as a 3D artist who has little knowledge of procedurals in a gaming environment, but I assume they behave similar to their offline-rendering bretheren. IMO procedural textures can be a usefull tool for adding variation to surfaces, and if there should be enough processing power to really do detailed and multi-layered procedurals in real-time one day, they could certainly also be used to replace certain types of surfaces that are texturemapped today (terrain, water, rock, etc.). There's also certainly a great number of special effects they could be used for, but in the end they will IMO always remain just an additional texturing tool and will never replace bitmap textures for most uses.
It really depends on they type of look you're trying to achieve, but in general if you want a near photorealistic look or need final control over the results (which is almost always the case), procedurals alone just won't cut it. Procedural randomness is in many ways welcome, but on the other hand is also has a tendency to immediatelly stick out as "fake" too. And while its often easier and faster to rough out a look with procedurals than using texturemapping, in most situations you just don't have the level of control you'd need. Last but not least there's the performance issue. Prodecurals are more computationally intensive to render than bitmap textures and performance would probably drop significantly as use of procedural increases. Just my 2 cents...
