Hard on the servers to calculate every game every time you open the game(Which I don't think makes much sense)
Wether or not it makes sense, thats actually the truth. Part of the reason that this is so hard on the servers is because every time you or your opponent's rating changes, so does this number, so in order to show it all the time, the server would have to be recalculating it all the time.
This is really no big deal, but I was wondering, why have the "Show Ratings Adjustment" under the details section of an Online game? Why not just show the adjustment? I only thought of two possibilities(while I was writing this...)