I have thought about this before, but to be honest I have never seen the point.
One reason is for trainers who have specific Pokemon, to battle them for EVs. Technically you would only ever need to know 6 of these trainers not every one. That information is a perfect question for Pokebase. Also, you cannot rebattle trainers in every game so it would be pointless for Black/White for example.
The other reason is for seeing every Pokemon and completing your Pokedex. Again this is a bit pointless as there is no reason not to battle all trainers. Moar EXP!
If you have any other reasons for a trainer dex let me know.