I also think 'A' teams and players playing 'B'/'C' is unethical... but we are not talking about that.
We are talking about a player who is learning/developing their pitching skills which are way behind her other softball skill sets - so at best a 'B' level pithcher. I have no issue with her playing on a lesser team to get circle time if she does not also use her 'A' skills to dominate the game (which is why she shouldn't bat or field - just pitch). If she wants to be a pitcher she needs circle time and I have no problem with her getting it this way.
I am fine with this as well. More than likely, said pitcher will not get any pitching time on her A team, so guesting for a B team may be her best opportunity.