I'm a huge stats geek so I love this stuff.    I think you have to add a couple more stats to truly judge the value of a player.  For example, batting with runners in scoring position and whatever.  I'm a big believer that players are worth just as much for coming up clutch/not screwing up a big play as they are for their basic offensive stats.  

Of course, I tend to think this way in terms of football which is harder to measure.