I don't see Boldin and Holmes as redundant because as you suggest receivers move around. Also you need 4 receivers not including TE's to contribute. With Boldin I would expect those 4 to be Sammy, Boldin, Holmes and Zay. I think Boldin takes huge pressure off of Holmes and in 4 WR sets they should benefit from each other and create issues for the defense.
For so long Buffalo just isn't used to a QB throwing for 3,500-4,000 yds - and I'd argue we just have not had the talent at WR to accomplish that despite of how someone feels about our QB's.
Maclin would have been huge and I hope he just didn't pick us vs not wanting to pay the money. Boldin is a very nice consolation prize this late and IMO should not be in the discussion of hindering any other receiver. Again, I think it's just because we haven't seen a passing attack in so long we don't know how to embrace it - we see it as cluttering the position and not letting young or FA options prosper.
To pass you need 3 viable WR options, one that contributes in key moments, a TE and a RB that catches.