I really don't think you need tendencies to get basic differences. The ratings just need to come out in the game. A 50 RTE WR should be running choppy routes, maybe even falling down like some of the WR did during the combine today trying to run the 10-yd out. "Controlling your speed and your body" is route running.
Those things just don't translate. You don't need tendencies to make technique differences show in a lot of cases. You just need to make difference in technique skill show up, imo.
To me, tendencies would be more impacting style/decision making in "ambiguous" situations. For example, a defender is about to make a tackle - does he wrap and tackle or does he try for the knockout shot, or does he focus mostly on trying to strip the ball? That, imo, would be something impacted by tendencies - effectiveness by ratings.
For a QB, if he scrambles, does he just try to buy time, using his quickness in the pocket to escape and extend plays and bait defenders into approaching him (letting WR uncover downfield), or does he scramble, see daylight, and take off upfield, maybe bypassing a WR coming free over the top to try to make the play himself?
An RB is about to get contacted - does he try to evade (and how? Juke, spin, cut, quick change in speed, change angle, etc) or does he invite contact and try to break through (and how? stiff arm, lower shoulder, push aside the defender, slight sidestep and then lower shoulder to try for better contact angle, etc)
Things like that would be areas I could see tendencies. That way two scrambling, fast QBs, wouldn't just play like 2 Michael Vicks, etc.
Tendencies like this could be good coaching influences. Let's say you draft a HB who probably should be more of a power runner, but he's trying to juke and cut too much. You could bring in a coach that teaches downhill running and perhaps start to change this kid's tendency towards his strength (what his ratings would dictate) and see his performance improve.
The problem isn't necessarily the ratings. Having 50 ratings doesn't mean that things like route running or play recognition differences shouldn't show up more consistently and apparently. If Welker and Moss look the same, that's not the fault of having too many ratings, it's that the differences between them in the ratings aren't being expressed (and maybe not being large enough - seems like the scale, effectively, is 80 to 100 unless you really suck at something - not a wide range). You could have 5 ratings and see the same issue.