If they need the practice, then practice. Beating up on inferrior teams only teaches them that they can play sloppy ball and still win.
In our small town, we have one select team made up mostly of girls who live here. They are B level. If these girls didn't play Little League, there would be no rec team in our town and the (about 4) girls who don't play select and do play rec wouldn't have a team to play on.
In the end I believe the rec teams benefitted by seeing better pitching, batting and especially base running. Monkey see, monkey do! They learn the most from each other and their opponents!
8U Rec. is a little bit different. Around here the players do not have a lot of options unless they want to play up an age group. I also do not think anyone has any idea what to expect. Like you mentioned a player practicing for a few months or over the winter has a good chance of coming in and dominating.
Another Team we played had a player that missed some games because she flew somewhere to try out for some sort of 10U national team. She could only pitch 2 innings so you just needed to survive it and get to one of their other really good pitchers.