Regarding the original debate. While I too have respect for players who are willing to show up at tryouts, I would not recommend it to young players.
Many events are scheduled tightly and it's extremely difficult to get extra players playing time. Also, information (programs, Scouting material, rosters, etc. are printed ahead of time for most "invite" only events.
However, I do agree that it's always possible you could find that hidden gem. There are many open professional tryouts and other events that are not invite only. I think players should try to attend as many of these as they possibly can.
We don't have many last minute add ons at our events, but we've had a few. We try hard to help them, but sometimes it is very difficult and they have to wait to the end.
One such pitcher, threw at the very end of our World Showcase one year. We had never seen him and most of the scouts had already gone. However there were a few still there and the kid ended up getting drafted the following June by one of those clubs.