We are disagreeing about something that is a matter of taste. You do not want bluffing in your game, that is fine, but I do. To say that it is a logical extention of my viewpoint that we should hide EVERYTHING is a non sequitur.
My post was intentionally over the top, apologies if it was too much 
It's meant to illustrate that our conversation takes place on a spectrum between complete information and no information, if we take your argument to its extreme conclusion we get a blank screen. If we take my argument to its extreme conclusion we get Chess or Go..
Showing mana doesn't strip bluffing out of the game at all - it *gives* you something to bluff about.
Why would every sane enemy just assume that? I've seen plenty of situations where I could make a reasonable guess after watching an enemy spam high cost spells.
Right now I have no meaningful data about your mana usage because many abilities can be triggered outside of my line of sight and remain effective. Or you can pop a mana pot and destroy any educated guesses I have made. Or we could be playing a team game and I was paying attention to your ally, not you. Or you could have bought 3k of mana items. See what I'm saying?
There are too many factors to control for, with current mana. That's why I got a little frustrated about the PAY ATTENTION narrative. 
As TBO says, I have to assume that you have mana. I attack, retreat, and coordinate actions based on that assumption. As a result I've trained my gameplay to essentially exclude mana.
Showing enemy mana means that I now have meaningful information to act with - I can't afford to exclude mana as a factor any more because it can be controlled for.
And this is where you can bluff effectively - showing me an empty mana bar and then popping a pot or restore when I fall for it will work now. At the same time, you will want to make sure you don't waste that mana, because if you do and you don't have any way to restore it I will make you pay..