I refer you to waffles answer over here:
Flag weight is now dead, you should decline flags that are not helpful and teach people how to flag in a helpful way.
Basically, people were not learning about bad flags because almost every single time a mod declined a flag, they would be berated by the person (ok, occasionally you'd get a request for constructive criticism, but they are few and far between), as the cost for a bad flag was massive compared the be reward for a good one.
I for one am glad to see it go, for the exact reason above. It wasn't worth the effort to decline a flag, because for someone like Iain, he had to flag 92 successful flags to undo one bad flag.
However, bad flags are also shown - in red in brackets.