It happens a lot....especially at the younger levels. When my son's friend's team would qualify for Disney at 10U - 12U they would pick up a kid or two...but would basically only use them as a pitcher (tourney could go 10 games) or to give a catcher a break (100 degrees is tough). Never pulled in guys to replace starters...and as far as I know, never had any complaints. We've brought in pitchers for a weekend every year that I can remember...but always either due to 1) a large number of potential games or 2)regulars couldn't throw for some reason. I guess we were good enough that we didn't feel the need to bring in "ringers"....but I know some teams aren't like that. I would hope that a coach would make his intentions clear at the start of the year. If he doesn't, then replaces regulars late in the year, I would definitely walk at the end of the year. Guys like that don't last long...and don't end up coaching quality players very long