game makers have a vision, and that vision is close to yours because you buy the game, but maybe not exactly the same.
with mods, there is always at least one guy out there who has the exact same vision as you and the skills to make it happen.
i do always play vanilla first, with one exception: minecraft, but depending on how easy it is to mod and what mods are out there i'll go for it. free content is free content, although sifting to find the gems is a bit arduous at times but definately worth it.
a good example of modding going right is kerbal space program, i recall reading some of the modders for that game ended up on squads payroll helping with the game. i hope the guy who made the russian rocket pack was one of them, he knows how to make rocket models.