I think the 'angle thing' is all the excuse you really need; you need every edge you can get. All the other stuff piles up as good reasons as well. I do know that a long time baseball tradition includes batting a lefty 2nd, to give an advantage to the base stealing leadoff guy in traditional line ups (as well as pulling the ball to the right side).
Baseball is lots of little things and all these reasons are cumulative.

As an umpire in upstate NY, I am starting to notice a lot more youth and legion teams are playing left handed catchers. I've been an umpire for about a decade now and I called more games in the last two seasons with a left handed catcher than I had in the previous 8 seasons. The availability of left handed catchers mitts probably has more to do with it at the youth level and particularly now as the cost of equipment has ensured that baseball is a sport largely played by relatively affluent kids that can afford to spend $500 a year on their own equipment. That said I think you will start to see more and more left-handed catchers in the prep ranks, however to see if this translates to the college or pro game I would think that Keith Law is probably right on the money given the premium placed on LHP. The last thought as an umpire is that a left handed catcher really screws with your strike zone behind the plate, particularly on the outside corner to a RH batter where he blocks a great view of the edge of the plate.

And we're back to where I said I bet pitchers would complain that LH C are costing them strikes.

But would you not say the Same about a right hand catcher against against LHB ?

No, it's the "comfort zone" for umpires.     They're accustomed to RH C.    LH C makes a completely different angle for them.    Not to say they can't adjust but, as an umpire just noted, it's more difficult. 

Mike is right, its not that you can't adjust it just takes more effort to make that translation. If you have experience behind the plate calls are often instinctive as you know your strike zone and react more than think. With a left handed catcher you have think about the call more frequently given the changes in the view and ball placement with framing. In my experience this causes me to think more and thinking tends to lead to missed or bad calls. If I am not sure if it was a strike instinctively then it is probably not a strike, the bad calls umpires make generally are calls to which they had to hesitate or think about before making them.

Another think to consider is most LH also bat LH, and if you have a solid LH bat most teams won't want to groom you as a catcher. You need extra days off because of the toll the position takes, and the injury risk is greater. If you have a good arm and stick, you're much more likely to be groomed as an OF.

http://www.nytimes.com/2009/08/16/sports/baseball/16catcher.html

Interesting story on catchers.