I don't have an opinion on Marshall vs Perine (or Kelley). I do think Marshall is an interesting player to keep tabs on though, and think the bolded above is overstated.
I'm curious where your opinion about him being a bad football player comes from since it seems to differ from other prospect reviews and footage I've seen (not claiming to be a great tape analyst here). We're stipulating that his measurables are close to, if not elite. The narrative on him that I've read and seems valid is that he started off hot and then dealt with a major injury, a followup compensatory injury, and finally got buried behind the deepest RB corps in college ball when he was healthy. During his freshman year, and during the limited carries he had his junior year he played exceptionally well.
I know... highlights. But check out the very first play. It seems like he has a decent feel for what's going on; no hesitation, quick cut to the hole, maneuvering through traffic, and then showing off that speed:
https://www.youtube.com/watch?v=CEDaTV4p28o (again, not claiming to be a scout)
Matt Waldman 2016 RSP: "Marshall can play in any run scheme because he has the speed and quickness to work the spread, but the patience, footwork, and power, to run an I-formation zone or gap scheme." In reference to his draft year season, "Marshall was a different back last year [2015]: He ran harder, ran from more compressed offensive alignments, mostly earned carries between the tackles and from the I-formation, and he faced more defenses that knew he could still bite them in the ### if they played only one safety high. Marshall played 11 games in 2015 and carried 68 times for 350 yards (5.1 ypc) and scored 3 rushing touchdowns." This is just an excerpt (hope you don't mind MW!), the rest is definitely worth a read. MW had him ranked as the 5th best RB, 4th postdraft, from the 2016 class.
If you combine that narrative with his ideal size, elite measurables, and the opportunity in WAS... it seems a stretch to completely rule him out. Dynasty owner here with more hope than optimism... just looking for info/opinions.