The best prospects should play in those meaningful games, if you want to measure them. You generally know who is a cut above the rest, from the Young Stars games.
But I'll give you an example of what preseason is. The Jets for their home game against the Oilers, dressed their top 3 lines, their 4th line C and 5 of their top 6 d-men.
The Oilers brought tryout Hoffman, Connor Brown, Janmark, Derek Ryan, Kulak to town. I guess Emberson was a d prospect. Maybe Dineen. Savoie and Sam O'Reilly are legit prospects. But what's the point of even playing that game? Fine tune your stars against 4th liners, 3rd pairing d-men and AHL/CHL players? Is that what preseason really is?
To their credit the Jets brought young stars to Minnesota and got bounced. But the roster disparities in some of these preseason games, like the Florida/Carolina game I mentioned is awful. I don't follow every other league's preseasons, but generally you go out there to compete?