I figured I should write my opinion, too.
Although I've always been a TTF lover [(TotalTTW+TotalTTL)/TotalGamesPlayed] and I'll always prefer that option when possible, I totally see why TD and CG went ahead not considering TTL (and thus TTF) anywhere.
Let's first say that I happen to remember that someone (Pella, maybe) said that FGei (it was back when it was called FGei, that's for sure) should be more about how much you win in an hour if you play the deck the best way possible. However, in order to reduce bias due to bad hands and stuff like that, a great number of games is also needed. Having a veteran (of that specific deck, some decks need someone who really knows how that works, see the Brave Malignant Ball that Could) play tons of games (which should be around 10 per God) without using the trainer (due to God's personal behaviour, although even with the Trainer it wouldn't be much less stressful) and on top of that making him/her take stats is just unreasonable. This is mostly seen when it comes to decks with a not-so-good UEI, because the player is even less interested in playing it.
This is when TTW vs. TTF kicks in. If it's not possible for a player to play that many games, having more players may just be as good. Sure, we'll be dropping the thing about using the deck the best way possible, but this would represent an average UEI with that deck. If you're a better player you may get a better UEI and win more, no complaints here. If you are under the average, no complaints here, you would have gotten a worse UEI anyway (and the gap would have been even bigger if it compared to a veteran).
However, now that we've dropped the best UEI, we'll have to try and remove bias due to multiple players playing the same deck. That's done via removing TTL. TTL is a nice thing to keep track of because it gets you TTF, which is a better way to calculate your UEI. Given that TTL is generally lower than TTW (not true for all of the decks, some of the times FG simply wins because of extra healing when he should have lost), this would rise the UEI. Removing TTL, though, means removing the subjectivity due to early dropping because of a bad hand or a bad turn of events (Decay could get a Fractal you didn't expect and completely quanta-lock you). This is why it's usually better to remove TTL.
However many players may say "well, there's subjectivity here and there, too, how about that?". We cannot possibly solve anything. The only way to remove the subjectivity (although still is subjective) is by making a veteran play and we're back to the starting point.
So, yeah, we're just trying to make it less worse.
TTL is, however, a nice stat to keep. If you don't mind adding that data, you can keep it for yourself and calculate your personal UEI with the deck, just for the sake of knowing. Remember not to include it in the data.
What's more, let me remind you what UEI stands for: Unified Efficiency Index. It's Unified what matters, we should really try not to differentiate from the UEI used for the various AIs and Arena. If a change happens here it has to be done to UEI itself, too.
TL;DR: TTW is the way to go due to subjectivity and stuff. You can keep track of TTL in order to get a PERSONAL UEI of the deck you're playing.
Hope this solved many issues and gave some more explanations, although I came late.
