I believe it is a big mistake that Ubisoft does not make modding an option in their games. When I look at other games like the Fallout series and the ArmA series, there is a huge following that does not appear to be subsiding. FO3 is a relatively old game, but you'd never know it when you look at the Nexus mod site. The interesting thing is that allowing modding greatly extends the life of the game because it creates continued interest.
Imagine how much game life would be extended in Far Cry 2 if Ubisoft had created a SP map/mission generator. I'd still be playing the game.
the X-Universe (X1, X2, X3 etc) is a UBI game and it allows for massive modding. I don't think that it is UBI's decision.
I'm not familiar with X-Universe - the only ones I like are the AC series and FC2, none of which can be modded :'(