Where do these pick up players come from? Don't they have their own team? How does their teams coach feel about the a player playing for another team? How long can a player be a pick up player before she has to pick a team? How does a coach explain to dues paying parents that their DD is being replaced for the weekend by a non dues paying pick up player?
There are multiple places players can come from. Our HC has borrowed players from the team of another HC he is good friends with. Their schedule allowed for a few players to help us out, but not miss any of their regular team's games. Our team has done the same for theirs, too. More recently, we picked up a player that had tried out with us in the past, decided to go another way, but then left that team. She was without a team at the time and was able to help us out for a weekend.
As a parent, I have no issue with pick-up players to fill in for missing players if it means the team can keep playing at a similar level. If it turn out that the pick-up player is a good fit for the team, and decides to join full-time, that's just fine.
In my experience, pick-up players aren't used to replace players who are already established on the team and present for the game. Pick-up players are used to fill holes.