I don't think this is a no-brainer. Gronk is the real deal but the TD count just isn't sustainable, not with the inclusion of yet another relevant weapon in Lloyd. Hern and Gronk's totals are going to be much closer this year than last. Hern is being used essentially everywhere on the field and IMO is going to push someone out of the top 2. As for your Welker assumption, I'm on the same page. NE just keeps piling on the slot WRs (just signed Greg Salas to go with Eldeman) and although they pale in comparison to Welker atm, I just get this feeling he'll definitely be playing for someone else in 2013.
When it comes to Bmarsh and Roddy, I think Bmarsh easily wins but is it by a large enough margin to offset BGE being in this trade? Roddy lead the league in targets last year 179 targets and was 3rd in 2010 with 167. We all know those target totals should come down by 30-40 as they transfer over to Julio. IMO, his reception total is highly dependent on volume and he should return to the 80-85 level that he enjoyed from 07-09. Regardless, he should still be an asset to any fantasy team. On the flip, I believe Bmarsh will return to the 100+ reception totals he enjoyed with Cutler for three years in Denver.
It looks like however Hillis will be your main RB3, and this trade should technically strengthen your core. You can never have too much RB depth but you seem like you can indeed make this trade. The selling point here is Bmarsh over Roddy but Hern over Gronk isn't the no-brainer it once was.
I just re-read your post and realized this wasn't PPR. Yay all that anlyzing when I could have just said, TAKE THE TRADE AND RUN! lol.