Back in the heady days of 2010 when the chat system was being refined, it definitely made sense to have user validation of chat flags.
At that point I'm guessing we had a dozen or so moderators across the network, and we needed a way to make sure anything questionable could be responded to quickly.
Fast forward a few years, and we have 300+ moderators in the network for 100+ sites. At any given moment there are at least 10-15 active-ish mods in the moderator chat, and more in their own chat rooms for their respective sites.
At this point, the negative consequences of having users review and validate chat flags outweigh any benefit that once existed.
It's extremely (some might say too) easy to get 10k rep network wide! If you have 200 rep any ANY site, you can associate your account to all the other sites and get 10k. (99*101 = 9,999, added to your original 200 -- you can now chat ban!)
Moderators have better judgement than random users. Mods use things like context to determine if a chat flag is valid or not.
There are so many 10k users checking flags now that actual mods don't get a chance to see them very often. I'm a mod on the DBA site, and I spend a good amount of time in chat, and almost NEVER get to see flags as they pop up. There are just too many users responding to them. This makes spotting potential behavior issues much more difficult.
In short, I'm pretty sure that the 300+ (mostly) elected moderators can handle flags in chat just like we do on the sites. We know the rules, we communicate with each other and the comms team, and are better equipped to manage chat and be exception handlers than random 10k users.
If you disagree, I'd be interested to hear some reasons to keep this feature in place.