I just did a quick check of Wilco (Williamson County) 5A/6A schools Brentwood, Ravenwood, Franklin and Centennial from the 2010 season. It looks like these four went a combined 3-7 vs. teams from Sumner and Rutherford -- all teams with a designated athletic period. Please check my math, I was in a hurry, but the numbers don't appear to indicate these 4 "do just fine" when going head-to-head against teams that lift during the regular school day.
IMO it's a big deal when the people you play on a regular basis can lift during the regular school day and you don't. The kids get to go home sooner, have more time to study or work part-time jobs. The off-season is even more important, because the underclassmen (those who might not yet drive) can lift during the day, then go home on the bus after school. It's certainly more attractive to the coaches, who can get home at a decent hour.