I rather use that spot on Monberaux instead of Ygnax most of the time, which is whenever I don't need the bigger heals too often enough (or if several party members are going to need a bunch of heals quick-like).
The fact that he can't be silenced is pretty neat, as is the endless pool of MP (so to speak), as well as the ability to give more of that to others... not to mention the area-like status effect removal abilities. Sometimes, like when soloing something like Ghatjot as a samurai, I'll take them both out to make things a lot more safe.
Then of course there is the Guard ability, which is a bit silly and funny but in certain cases, a little bit useful too!
At first I was not too impressed myself, but I imagine he is the most used alter ego for me since a long time ago now.
As for the "Monberaux tax", I guess they could have opted for different amounts for different levels of characters instead. Would that be more fun?
Perhaps they just wanted to have a kind of a realness to it, where the more wealthy can donate more, and didn't opt for a static amount. Yeah, it kind of becomes silly when you can just store the current gil away and only pay 10k anyblue.
Mayhap they should consider the box in Mog Gardens as well! ^^;
(Please don't.)
I wonder if it would be nice to have something like, the more you pay, the longer you don't need to pay again. Say, maybe 100k would do for a month or something like that.