I've seen cows with really bad mastitis have that teat cut off on purpose, so it can drain. Last resort, mind you, and it's not a pretty sight.
After watching several of those cows be dried off and then calve and enter the milking herd again, I note that the udder tends to become lopsided. As a dry or late-lactation cow it isn't quite as obvious, but in the week(s) prior to calving the side where one teat was cut off can swell two or three times the size of the other side. It doesn't "behave" like a normal, four quarter udder where all four quarters swell and fill with milk the same amount.
On the other hand, a cow who had mastitis in one quarter but eventually recovered, teat left alone, and the quarter simply does not give milk...that udder can look perfectly normal.
I'd be more inclined to leave that teat alone, regardless of if it gives/will give milk or not, because IMO her udder will hold up longer with four teats on four quarters. Which means longer productivity, longer herd life, etc etc.
Just my opinion.
i usually dont let things like this get to me, but it did this time... wasnt expecting what i found.. being cut to the milk canal, well that kinda had me reeling there for a few minutes.. it came into perspective when i remembered that cow i had that almost had one cut off.. 
