I'm a big fan of summer leagues. I find myself agreeing with everyone.
I agree with Consultant that they provide irreplaceable experiences, and not just about baseball, for very little cost. The last two summers my son has paid nothing - I view it as an unpaid internship. Playing pro ball is not the only job in baseball, after all. I also agree that training in a gym is not the same as playing games.
I agree with Absorber that many of these teams provide live outdoor entertainment centered on a baseball game for a community, and that is very valuable. My son has played on teams where people love to come to the games, and that provide good entertainment (and good beer). This summer I randomly heard a mom say to her young son "there are 4 baseball games while we are here, we can go to one" - that's just fantastic. I wish we had such a team in our area.
I agree with nycdad that paying $800 for the experience is not something I would do. The leagues that make money by providing local entertainment is one thing, but glorified travel tournaments doesn't make sense to me.
I also agree that there is a variety of players who benefit from playing in summer leagues, such as those Absorber mentions. The transfer portal has resulted in players leaving their summer teams as soon as they get a new college team, as Hokieone noted last year, but on the other hand, perhaps it has provided an inducement for players hoping to transfer to play.
Finally, I agree with d-mac that the level of players is not what I expected, for the past 3 summers. Some of the baseball has been surprisingly good, some of it has been surprisingly bad. I agree that top-level pitchers are not coming; top teams' starters are pitched out (and I agree they should not play in the summer), but the relievers and bench players don't seem to be coming either.