I've said this a few times before, but it bears repeating: Phoenix was one of the worst teams in the league last year. Marshall was a lottery pick. Yet, somehow he couldn't manage to carve out any playing time for himself on the Suns? Hell, as someone noted, Ish Smith (who was undrafted and is really fortunate to still have a job in the NBA after three seasons) was retained in favor of Marshall -- and Smith was a throw-in from Milwaukee in the Caron Butler trade. Consider the fact that Marshall also apparently has a bit of an attitude issue and I just don't see the appeal in the guy. He's a pass-first point guard without any other NBA-level talent.
I'd argue that a guy rejected from a team that seems to be bad at talent evaluation isn't necessarily a guy who is worthless. For example, the Cavs waived second-year player Danny Green in October, just before the start of the 2010-2011 season. The Spurs signed him, waived him, then re-signed him a few months later after some time in the D-League. The next season, he started more than half the team's games.
It is not necessarily a strike against Marshall that the Suns have been looking to trade him since last season. It is a strike against him that new Suns GM Ryan McDonough (credited with pushing the Celtics to draft Rondo and Bradley in his time with Boston) didn't feel like keeping him around as part of a rebuilding phase.
I'd probably be more interested in gambling on a draft bust from a poorly-run team than a draft bust from a team with a good track record for evaluating talent.
Marshall seems like a guy who I wouldn't want to sign right now unless it was for an unguaranteed contract that I could easily waive, but I'd probably keep tabs on him if he went to the D-League.