Ward was one of Waldman's favorites in the UDFA class and he said to stash him at some point if possible and if there was reason. He liked his speed and pass-catching ability. "Ward is an excellent fit for the spread scheme in Arizona, because he's an excellent route runner and pass receiver from the backfield. He has enough speed to break plays, runs with determination, and he fits the mold of a scat back with enough heft to work with. He'll compete for a final roster spot with a good showing this summer/fall." Well, he's off the practice squad and has been for a bit.
As far as why Edmonds and Benjamin don't strike me as three-down guys, I guess I just have a hunch from watching Edmonds and Benjamin's highlights. Waldman was high on Benjamin. I saw a painfully average back, but highlights can be misleading. I may be missing virtues to Benjamin's game that Waldman can see a little more easily. I don't really think he's that fast. According to RotoUnderworld, he ran a 4.57. He posted an above-average 11.22 (71st-percentile) Agility Score, but his 4.57 (56th-percentile) 40-yard dash resulted in a below-average 94.9 (44th-percentile) Speed Score, and his lackluster twelve reps (4th-percentile) on the bench press contributed to his 105.6 (33rd-percentile) SPARQ-x score.
I didn't see a great athlete in Benjamin and this appears to confirm it. Ward ran around a 4.53, according to sources, though nothing concrete can be found. That said, Edmonds ran a 4.55, so they're all clustered around the 205 lb. range with similar speed scores, it would seem. Edmonds had a 95.5 Speed Score with an agility score in the 97th percentile, so it seems like he has more burst than Benjamin. He also benched 7 more reps.
So back to the three-down thing. Call it a hunch. AZ plays a spread offense, so you wouldn't expect its back to be a pounder; rather, you'd expect someone adept at catching the ball and running the read-option, which Edmonds is. But a three-down back? I don't know. I'd expect to see a rotation with the two guys, maybe with Ward rotating in once a game.
Really, I'm just spitballing. Take this information with a grain of salt, really. I roster both Edmonds and Ward in dynasty now (bid went through for a buck tonight) so I'm hoping that one of the two takes it by the throttle in the upcoming year or so.