Given Slaton's inconsistency and Marshall's potential decline in value if Orton misses any time and/or doesn't play at 100%, I'd say this is a pretty good trade.
Wayne is arguably fantasy's #1 WR this year, and is a definite upgrade over Marshall. Combined with AJ, Moss, and Jennings, it gives you a solid receiving corps, as you can now start Wayne and AJ, and then use Moss as a flex play.
And given that you have Benson and Jones as your primary backs, you likely weren't using Slaton much to begin with. Assuming you can pick up a good waiver-wire option as security for Benson should his injury turn into anything significant (e.g., Scott, Charles, Betts, Wells, Forsett), I say do it.
Again, the only issue I'd have with this trade is that it leaves you precariously thin at RB. Pick up a good back-up off waivers, and your WR's will likely carry you into the playoffs and beyond.
Edit: If you go ahead with the trade, you may want to then consider turning around and trading away Moss or Jennings for a solid RB. You'd still have good depth at WR, and your RB depth would then improve significantly as well.
(0.5 PPR, 12-team league)
QB: Vick, RG3
RB: McFadden, Matthews, K. Smith, R. Williams, M. Goodson
WR: Nicks, S. Smith, Britt, LaFell
TE: Hernandez, Pitta
DEF: Spot start (Cin in wk4)