Mike - My DD's team played primarily an ASA schedule, with the intent of college exposure. There were three showcase tournaments that nearly every NCAA and NIAA college coach in Ohio and surrounding states regularly attended. In the interest of college recruiting, there was no need to split the first year 16s into a 15u age group at these tournaments. As a matter of fact, a 15 year old kid playing a competitive schedule with a mix of 16s is far more impressive to a college coach. They want to see their potential recruits tested. IMO, the even numbered age divisions have worked very well for this purpose. While a few powerhouse teams do play up, it's usually at less competitive tournaments.
U-Trip for baseball is another story. 15u is very common in baseball, but there are FAR more teams in travel baseball, and the talent spread is much closer than in fastpitch softball.
I applaud you for providing a venue for girl's fastpitch. It's a TON of work, and the more outlets there are for the girls to be involved keeps them from straying to the "dark side".