My opinion on this is:
I would not run that setup unless I need to, like with my TWM ITBS where the idler pulley cannot clear. The cost to run this setup is the same as the idler pulley.
The reason I am not crazy about the 3 pulley setup is that there is not tensioner... so replacing a belt would be a bitch. Supafly run this motor for 20K miles.
As far as less rotational mass, I would not worry about it. The loss or gain of power, is non existant in my opinion