it prevents this...
dd has played four years with... (Classics, Lasers, Doom, Extreme, etc. etc. (you name the team here))... at 13, the team finish 2nd at States, and finished 36th at nationals. At 14, this team has a chance to really rock.
dd shows up at high school... will probably be 4 year starter on varsity...
high school coach walks up to dd and says... we have a travel team that plays here at the high school. you are expected to play on it... "but coach..." she says... " i have a travel team"... "not any more... if you want to play varsity here.". Joe Smith... my brother coaches our travel program and every girl that plays varsity plays on this team... meaning... you want to play varsity... you better be on this team...
personally... i like the 4 on a team rule... it protects the girls from the example above.