I think guys like Montgomery are valuable for a team. When they're on the field, they can be lined up as an RB, wide out, or in the slot. It keeps defenses from knowing whether to play pass or run, and changes what their personnel packages often are.
That said, it's a luxury having someone like this. Over the years, so many of them have been around, and usually they're built more like a WR than an RB. Therefore, a little more prone to injury. Evidenced by Cobb, Montgomery, and Percy Harvin, who was with the Vikings, Seahawks, and I believe Bills?
They probably don't represent the #1 player in any singular position, but do make for value when it comes to filling two positions or more, as a back-up.
The most interesting multiple position player that I see now is Tanner McEvoy from Wisconsin, playing for the Seahawks. He catches passes, plays CB and safety, and takes some snaps at QB in practice, because he's there, and can do the job, if asked.
These guys are more like the players prior to the 60s. There was a time you played both ways, and were only allowed so many substitutes per half, and when you did sub, guys weren't allowed back on the field. It was a screwed up game for quite a while to be honest.
