I'll hit a few points here.
First off, I think a lot of people overvalue a PG grade. I've seen many instances where a player and/or parent act as though a PG grade is the be-all and end-all form of determining their future(s) in baseball. PG events aren't an absolute necessity for getting recruited or scouted.
With that being said, PG is an absolutely outstanding venue for such exposure, and is easily at or near the top of the "game" in doing so. Going to a PG event, if capable, is a very good experience and often times will lead to exposure. I am very much a fan of PG events and I know that many college coaches and professional scouts attend several of them as well.
I can't speak for other talent evaluators, but I consider a PG grade a good piece of information on a player. I wouldn't evaluate a player based off of another person's evaluation. I trust my evaluation skills more than anyone else's evaluation skills, simply due to the fact that it is my name going on that evaluation.
PG is a very good resource and the people running the events do have a good sense of talent. If I see a player with a grade of 10, I will probably glance through that player's profile page and get a little more information about him. Same with a player that has a 9, 8, 7, 6, etc. I have been to PG events and seen a player with a 10 and felt as though that player was not a pro prospect, and I have been to events where a player has received a 7 and I thought he was a pro prospect. Beauty is in the eye of the beholder!
So, take this with a grain of salt. PG evaluations and grades are a nice tool. I consider them a snapshot of a player's abilities, with an evaluation to project a player's future tools from someone who has seen a lot of players and has a good sense of what they're looking at. I would never turn down information about a player, and PG provides a lot of information on a lot of players. I wouldn't evaluate a player based on another person's word, regardless of who that person is.