Late to the party
I would support a winner-only method of looking at stats for several reasons.
If the argument is that the rating approximates the same thing as whether you win, then that would mean the two methods are no different. If the two methods are, in fact, almost the same, there is no reason to use one over the other. Only using the winner's stats seems to me more straightforward, which means it has an advantage in being understandable to people who aren't mathematical, which I would say is an advantage.
However, keep in mind that Pokemon players use multiple teams. The rating can be seen as an attempt to measure the "true" rating of the player, but the true rating will vary based on which team the player is using. A 1300 player with one team might be a 1400 with another. Using only stats from the winner will take that into account, as well.
It seems that winner-only stats would be a direct measure of what usage stats for tiers are apparently trying to measure: Pokemon that win. Why use a substitute when we can just directly measure the real thing?
Also, if there is still interest in trying to re-build a team based on usage stats, I have a program that does that very thing. I'm using it as my team prediction algorithm for Technical Machine, my Pokemon AI, but it has the effect of building a team based on any number of Pokemon by 'reversing' the team stats.
, a Pokemon AI.
"Strategy without tactics is the slowest route to victory. Tactics without strategy is the noise before defeat." - Sun Tzu