I would guess that would be way too much work - unless they *knew* from a source inside the FBI/cops that they were being watched and could set it up perfectly. I would think most of their efforts are put into not being watched/surveilled at all, ideally.
It's hard enough to carry out complex criminal plots never mind trying to carry on a parallel plot just for show. I can't imagine the FBI would invest much time following around "decoy" associates who never end up committing any actual crimes. And without evidence of any actual crimes they're not going to end up with warrants, etc. to take the investigation very far.
I'm sure the mob is happy enough to take advantage of "useful idiots" for whatever different purpose, but as a pre-ordained plan I would think its not plausible.
Yes if they knew who was being watched in the family the decoy would be used to take heat off of them and throw off the feds and confuse them. I would think this has been done before, all you have to do is look at FBI Charts and realize that some of the guys who are running things dont' even make those charts in the first place.
Lets create a scenario here, where a group of men show up for a sit down. The crew decides they are going to send in a couple decoys because they know that the place is under FBI surveillance.