It does depend on the kid. The skills built from playing travel from 3rd thru 6th are really important from my experience. Now that my daughter is older, she doesn't need to play full year, only 6 months to maintain those skills - while playing other sports in off-season. If you want to make travel teams in 6-8th grade, the girls need to know the rules. I find that girls that don't play full year early simply don't learn the game. They only play March-May and don't pick up a stick for 9 months.
Taking a 3rd grader on road trips gets costly. If they stay on Long Island it is worth it, especially when most organizations don't charge you the same fee as older girls.