There have been many memorable mods that have delighted fans, like the immortal Team Fortress that graced Quake 1, to the ingenious AirQuake for Quake 2. The public SDK tells everything one need's to know. Gone are the fiddly file configurations and command lines that were needed to activate the mods of yesterday, as now with most mods one need only create a folder, unzip to it, open Half-Life's Custom Game option and hit Activate. Whilst multiplayer is generally thought of as the de facto way of achieving a game's longetivity, mods have certainly paid of as well. It's obvious that MODs have become too popular to shutdown or reduce.
MODs benefit the developers because it makes their games last longer in their usage.
Promoting the ability to develop these MODs will only help them. If multiplay is your cup of tea, then you have dozens upon dozens of MODs to choose from, and hundreds of player made maps. Well, we can't exactly measure how much people play online vs how many prefer single-player, but one day it will become heresy to make a game that can't be played online. Single-player mods end on the last level. Without mods to extend the flexibility of a game, the game will die.