Not really, (+/-)12 is not the quotient here, but just the result of the calculation, and it reflects the difference in player's "strength". In the case that both players have the same rating (points), it is 12, but say Federer had 2890 points, and Tomic had 2000, then if Federer beat Tomic, he would gain 24 points, and Tomic would loose the same. But if Tomic beat Federer, then they would both gain 0 points, that is, they would both retain their previous points. Thats how calculations in Elo system work. And the point is not to rate tournaments as important or unimportant, the point is that wining is awarded the same way on any tournament (between the same players).
The difference in points reflects probability that one player would win over the other, something like determining the betting odds. Rating system is in a way cumulative.
What I would like to discuss, is how to grade a result of a match.
In chess, there are 2 outcomes, win/loose and draw, and win is awarded 1, draw 0.5, and loosing 0. In Elo rating calculations, result should map to a range between 0 and 1. In tennis, there can be only win/loose, so simple way would be to award 1 for win, and 0 for loosing. But, I am thinking of a finer grading, that could reflect more subtle differences between player's performances, and also establish the difference between matches on grand slams and matches on other tournaments (winning in 3 sets vs. winning in 2 sets).
No it isn't. Take it from me - I have played competitive chess and have an international Elo-ranking myself. If two players ranked 1500 meet and the winner gains 12 points, then we have the following scenarios for players ranked 2890 and 2000. If 2890 wins - plus one point, he goes to 2891 (not precisely one, I rounded off). If 2000 wins, plus 23 points, he goes to 2023. As you see, winning over a higher ranked opponent is worth much more than winning over someone lower, or your own class. But winning over a lower ranked player is never completely worthless.
In theory, there is no problem calculating more results than two or three. But I don't see what winning in three sets should be worse than winning in two. In chess, there is no bonus for mating the opponent in eight moves. You get the same score if you force him into submission only after 100 moves.