I think nowadays it would be very hard to argue against the value that it adds to games, which makes it more attractive to gamers, which translates to profits for devs. Between games with and without mod support, those with it play and sell well beyond its normal life expectancy. ~ Norbingel
Why cant Second Life open for mod support? This would increase creativity and tempt a lot of mod enthusiasts to work on making the platform even more exciting. You could have different mods on different sims and it is possible to block mods too if you dont want ppl to use them. You can both use mods that are implied on a specific sim (like weightless) and other mods as for example graphic mods in the viewer.
Examples of mods:
Weightless in space; a mod that removes gravity or decreases it
Graphic mods: Look at ENBSeries: http://www.enbdev.com/description_en.htm
Car physics mods: Make vehicle physics more realistic
Ragdoll: If a person or animal falls uncontrollable. Look at this example from Skyrim: