I haven't gotten much lately, but when you get one - it's just not something that is productive in any way.  

Many others have suggested it over the years. The problem is I don't think TER figured out a way to allow people to ignore aliases without ignoring the handle the alias is attached to. At one time they tried and that basically outed people's handles. There has to be a way to ignore the alias without tying it to the handle.

It's "only" a matter of, most likely, a database redesign (really big $) and then a SMOP (could be big $).  I expect the cost is why the change hasn't been made despite many requests.  

even if they didn't do anything wrong, just to out it. That could back fire.

Right now the "ignore" feature couples the PM system with the discussion boards. IOW when you hit "ignore user" you not only block that person's PM's but you also ignore his board posts, THAT of course is what makes it so easy to out an alias by using a simple method of trial and error to figure out what any given alias a poster is using

I think the number of PMs would taper off greatly.

They might as well change the system to handle this. It would likely cost less in the long run than a manual exception process, and would be more consistent.

And the upkeep / tracking would be ridiculous. This is a reason I don't like DM's or private emails. It's a huge cluster-fuck of confusion sitting and talking to Aliases. Not that it's required that we answer, but... it would just be nice to press "ignore" and be done with it - nobody in the middle.

So it seems that this has several moving parts. User ID - Alias and then PM from User - PM from Alias.

... but there would be coding involved, plus some database changes to support it.  

Not sure about the need to DB changes -- other than perhaps some triggers which I'd lump into code changes. Then again, I don't have a clue what the database structure looks like so....