Having experience running a forum dedication to mods of a game (warcraft3), I have the view that scenario editor mods generally can create really good stuff.
How good? Depends on the amount of people and the skill of any particular individual. I've seen some that can challenge professional level maps, and even beyond. There are crappy ones, to be sure. People attempting to play around and all that. Everyone's a noob at the start. But like blogs, most suck, a few are interesting, even less engaging, and a precious few, a total success. (refer to DotA and HL:CS)
The game would be of a lesser value without it. Games with good modablity tend to have a thriving community which builds branding and brand loyalty(hope the CEOs are reading
), not to mention theoretically increases the replayability of the game. Replayability which effects any buyer of this game including you and me, including others considering purchasing it.
But in line with Gatts's response:
Would I welcome scenario editor in KB tL? - Yes
Would I miss it if it would not occur? - Yes
The reason i would welcome it is already stated. The reason I would miss it, is because it could be the next Counter-Strike of the TBS genre (this is consider a TBS rite?), one which is long overdued. And it would be a pity to miss such a thing.