Hi Dev, yes there is a benefit to hooking up the manometer, even without a barometric damper.. the manometer will show if you have way too much draft or not.. like coaledsweat wrote, you will be able to see if you are wasting coal and heat.
What I suspect you will see is that when it is windy and cold, the draft will be quite high, that is in excess of .08" maybe as high as .15" wc.. and at the same time, your probe thermometer in the flue pipe above the stove will show a big increase in temperatrure. This means that with the excess draft, the coal is getting excess air pulled through the coal, and this heat is not staying in the stove, but being pulled out of the stove and up the chimney..
With a barometric damper, you would have a steady draft, the baro would open allowing some 70* room air up the chimney, but the really hot, 800*-1200* air would stay in the stove, where it can heat your room.. and your fan can wash this heat off the stove body.. Since you have for many years gone without a fan, you probably have had a very hot stack temp for a long time.. the distribution fan and a baro will get the most out of your stove..
But the interesting thing about this experiment, and why I sent you that manometer, is that your coal use for the number of square feet of house you are heating is very low already.. so my curiosity is high...
Do you 'tweak' the combustion air control to the fire every several hours?? or do you have a "set it and forget it setting"?