I certainly don't hold it against a player (or his family) when he is asked to be a guest player. They aren't responsible for dynamics on the team.
I suppose it comes down to how teams/coaches set expectations. Bringing in guest players who have already committed and having them take innings from kids on the team who have not yet committed could be justified as helping the team go deeper in a tournament and thus get more looks... Or not. Depends on the situation, IMO.
If we step back and think about it, the travel team tournament system is pretty odd. I don't think that any other level of competition--Little League, Middle and High School, College, MiLB and MLB--allows teams to add to their rosters essentially without limits. Imagine if the Dodgers could beef up their roster for the NLDS with a few players from the Rockies, Giants and Diamondbacks? Why not call up Bumgarner and see if he wants to pitch a couple of games for L.A. in October? That would be very, very strange.
I'm not complaining: travel ball is different and we know the rules when we sign up. But it is an unusual system in a lot of respects.