As long as the "subjective" ratings are based around a scouting system (like in OOTP), I'd be cool with that. Add another slot in the scout team for the current players in the league (i.e. not just scouting undiscovered talent), and it sounds like the same sort of thing. The value arrive at for a player is based on the scout and your own observation, just like the stock market (analysts, company reports, your own instincts and trading personality).
Just showing me randomly "wrong" ratings so I can make mistakes or leaving me with zero information on a player - that I wouldn't find realistic just like knowing exactly what a player is isn't realistic.
If I have Evan Longoria, I know what he is or isn't. So do the Rays. Putting him as a "blank page" wouldn't be realistic. For someone like Wil Myers - more "estimation" based on what the scout thinks the player will be given the scout's own tendencies, abilities, and biases.
Something like that I would find realistic. There's not a whole lot of mystery around guys who have been around (who doesn't know Rivera was a top closer?), done things, etc. Around prospects, minors players and guys that haven't been in the majors long, definitely.

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