I'm unsure whether I prefer daily or gamely uncertainty decreases myself (or both, like Goko does).
Correct me if I'm wrong, but if I remember the TrueSkill documentation correctly, the per game uncertainty models the idea that the skill you play with for
that game is itself drawn from a distribution. One never plays with a fixed underlying skill. For example, I may be watching tv, be under the weather, distracted by something outside, etc. These are factors separate from luck within the game itself. [The modeling assumption is that the parameter (beta, I think?) that describes the distribution from which you "draw" your skill every game is the same for every player.]
The daily uncertainty increase is an artificial way of 1) encouraging play and decreasing leaderboard camping and/or 2) crudely modeling skill depreciation over time.
I do agree with michaeljb that the rating shouldn't drop after win, but that's a "bug" of TrueSkill, which Goko (and Isotropish) just copied. Ideally, you'd limit the automatic uncertainty increase by the mu increase (to give at worst a rating change of zero) in the case of an expected result. "Lying" about the rating decrease doesn't help people who get stuck with an ever-decreasing rating for continually beating Serf Bot (there was such a case last year in Casual mode: http://forum.dominionstrategy.com/index.php?topic=6819.msg189088#msg189088 ).
Edit: added link.
Again, ranking systems are not achievement systems.
mu - 2*sigma is just one way to represent a two-parameter system as one number to create a leaderboard. If you prefer not having a rating decline purely because of uncertainty increase, then consider preferring a leaderboard based only on mu rather than changing the system itself.
Ideally, the leaderboard/rating decline from a win against a weak player wouldn't necessarily impact the quality of matchmaking, either. I think Microsoft tries to match players on the highest expected probability of a draw (not the same thing as closest rank on the leaderboard). With good matchmaking, rating declines should almost never happen, anyway.