Because that thread went 600 comments and it's easier to sum up the points made for future reference than to reread the thread.
Crawford: .353 wOBA projected rest of the way
Joyce .327 wOBA projected rest of the way
League average wOBA: .328
Over 300 plate appearances, the difference is ~6.5 runs.
Crawford 9.9 UZR/150 in LF (736 INN)
Joyce career 12.5 UZR/150 in LF (409 INN)
Joyce probably isn't a 13 run defender over 150 games and even if you think he's average, the difference is about 3 runs over the rest of the season.
6.5 + 3 = 9.5 runs, or roughly a win. That number shrinks by the game.
The main arguments for/against
- Helping team in the future is worth the difference this year.
- Payroll strain would be eased.
- Other aspects of the team regressing, like the rotation, might make the Crawford less negligible.
- It's Friedman, he's not getting hoodwinked on a major piece.
Do not trade Crawford
- Perception. People may stop going to games/buying jerseys, etc.
- Trading present value for potential value. In a tight division present value means a ton.
- Potentially trading positional players for pitching prospects.
- Choosing prospects over draft picks.