Problem is that range factor is hugely dependent on other factors besides a player's real range. Handedness of the pitching staff causes more balls to be hit to one side of the field or the other. Groundball/flyball ratio of the staff changes infield and outfield chances. A gap in the quality of the outfield compared to the infield (or vice versa) can subtly change the number of putouts each gets.
For first basemen, 90% or more of all putouts are just catching a throw from someone else, standing on the bag. Assists are dependent on whether or not the guy likes to take it to the bag himself, or just wait for the pitcher to cover. For catchers, most putouts are just catching the third strike. For third basemen, putouts are highly dependent on how much foul ground there is in his home park.
There are ways to compensate for all of these biases - the newest work from Baseball Prospectus and Bill James each do a good job. But the methods are complex and time consuming. Subjective analysis may be the easiest thing to do now, but in a month or so James' new Win Shares book comes out, supposedly with defensive analysis of every player in history. That could be very valuable in rating real range.
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