Reserve teams weren't really an experiment, except here in the US. Most of the world doesn't do "minor leagues." They do promotion and relegation, and cities that would have no control over their players in a minor league system HAVE control over their players in a lower division.
The thing with reserve games... because you're only allowed 3 subs in a game, at least half your roster is guaranteed not to play in a given week. Theoretically, they need more real playing time. That's why England used to have a robust reserve "league" of sorts, usually playing Monday afternoons after a Saturday game. As it stands, they only play about 12 games a year now.
I know what generally happened in Portland: early in the season, all the reserves are healthy and the early season reserve games were fully stocked, so to speak. As the season wore on and more players took knocks, any trialist not necessarily on the roster would get a chance to play, and by the end of the season, there might be 4 players from the U-18 and U-16 teams playing. Last year, in Portland's case, starting.
In MLS, the roster limit is 30. Elsewhere, generally, there's no limit except what you can afford (in some leagues, notably Germany, going into serious debt will get you docked points in the standings). Of course, in most of those leagues, their U-18s and maybe U-16s ARE signed to professional contracts, though the younger ones are usually "paid" with an education from the team's academy. Teams like Manchester United might have 60 players under contract, many of them loaned to other clubs.
One more little tidbit: I'm not sure if USSF has always had the "cap-tied" rule like other countries, but they do now. I'm saying having the B-team in the cup is impractical simply because of the way the players are used for the club. More important, say there's a trade. If you played for Portland in a cup game and get traded, you cannot play for your new club in the same year's cup competition; you're considered "cap-tied." Now, imagine trying to get the B-side through those hoops. It's no wonder most teams are probably sending 2-5 players to USL PRO teams and calling it good. I'm curious to see if Galaxy II are in the tournament... maybe? Only because there's likely plenty of players not on the 30-man roster, especially if LA is smart.
However, next year, NY Red Bulls have announced they'll have a full team in USL PRO, and rumors point to Seattle doing the same.
We have always had a cap as far as i know.Soccer has been Americanized here.We believe in fair play and feel that the clubs should have a equal balance and thus we have a salary cap. I'm thinking that even though the Galaxy 2 for example is considered a b team there cold be a argument that they are a separate entity from the the MLS galaxy and thus a separate team. I would think they would be in the tournament i would think they talk with the coaches or management and tell them to hold a couple players on the bench.