1-mods normally fix bugs and extend the life of a game
2-People here love them some mods
3-the extremely active modding communities for Oblivion, Morrowind, and Fallout.
and
4-they're clearly an "intended use" of the game, since Beth made it so easy (both via file structures and the release of the GECK) to mod.
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as to the other part...... different game genres, I'd rather get on different systems.
Platformers, racing, JRPG type games? Console.
FPS's, RTS's, strategy, MMO, western RPG? Computer.
Some things just work better with keyboard/mouse, and others work better with a good controller.
Would not be able to stand Morrowind/Oblivion/FO3 on a console. /brrrr