I think it's largely a myth. I was a head coach in Georgia for 4 years, the last 2 in 12U, and I'd describe us as a B+/A- team, but I didn't experience it as a conundrum. There were plenty of B+/A- tournaments. They weren't necessarily called called B+/A-. Some were A-level tournaments with mediocre A-level teams in them. The Vipers and Impact didn't enter those. Others were B-level tournaments with at least a couple of really good B teams that were going to be hard to beat. We did enter tournaments where we knew we'd probably get run-ruled at least once, and we played some where we thought we'd win and did, but we didn't have trouble finding something in between.
A good B+ team will beat most B teams and lose to solid A teams, so the coach has to decide if they play B and win, or play A and lose. A tournament with only B+ and A- teams would be very competitive and can happen, especially at 12U when the number of TB teams peak and there are multiple tournaments every weekend to choose from. GA is blessed with several organizations that put together weekly tournaments so it is not unusually for a coach to decide on Wed or Thur where they are playing that weekend. When we started playing ASA qualifiers, the tournament registration closed 2-3 weeks before the tournament, so you have to decide much earlier.
When my DD's team was a B+/A- we loved playing tournaments like the Carrollton Classic where you get a 5+ game guarantee, so even if we played up and lost we still got 5-7 games in a weekend. Compare that to an ASA qualifier where there are no pool games and it is very possible to go "two and out".....