Well, if tournaments provide us a good live scoreboard via the internet, we are not only given point by point scores, but sometimes even statistics!
The same is also true for most TV broadcasts when graphics with several stats are shown.
So what are, in your opinion, the most useless statistical numbers or categories?
For me this is "2nd serve percentage"!
I really don't give much for knowing if the 2nd serve percentage is 97%, 100% or 94%. It's enough for me if double faults are counted.
Also I think that the "unforced errors" category can be useful sometimes as a rough indication who the match is going, but it can be also a very controversial one as this is a very subjective category because it can depend on what people rate as either a "forced error" or an "unforced error".
A perfect example of how fucked up such a category can be was during the Olympic games as the official Athens (or NBC site) counted about 50 to 70 unforced errors for most of the players in almost all the matches. And sometimes it wasn't even mathematically possible that one of the players made so many unforced errors because the number of "unforced errors" was higher than the numbers of points they lost