"I am sure that even BB/SP would admit that they did not play their hand well this year."
I don't agree with this statement at all.
Bill and Scott Pioli did what they did because of several reasons, and I am sure that they were fairly confident that they would be able to sign Deion Branch. If they attempted to sign one of the big name receivers that were available last offseason (overpaying for their services), they would have created an even bigger dilemma when it came time to negotiating with Deion, who would feel as if he deserves more money than the recently acquired player.
If the Pats had eventually traded for Javon Walker (which they were pretty serious about, Walker just never boarded his plane to Boston) they also would have had the dilemma of restructuring his deal. Walker had also pulled the same Branch BS last season. After going to the Pro Bowl in '04, Walker wanted "his," and held out of training camp to show the Packers who had the leverage. They didn't budge (good move) and he eventually came back (and got hurt). He wanted big time money, and, while I don't know the contract the Broncos gave him, I can bet it was more than the Pats would have given (see also having to eventually pay Deion).
The Pats signed Caldwell to replace Givens, and he has done so (production wise), but they also had every intention of playing this season with Branch (who forced himself out of town). They drafted a "stud" WR in the second round, who unfortunately got injured. Keep in mind that many prognosticators felt Jackson was a steal and the best WR in the draft, a true first round talent.
After the realization that the Pats were not going to have Branch for a good deal of the season they attempted to sign/trade for another receiver, but how many receivers were out there? Donte Stallworth would have commanded a draft pick and a new contract if they expected to have him available for more than just this year.
Looking back on the situation now and saying "they could have had this guy" or "they should have tried harder for this guy" is hogwash and a classic case of hindsight being 20/20. In doing so you also fail to realize the situation the team was in at the time those players were available. If Deion doesn't pull his BS or if Chad Jackson doesn't get injured this season is entirely different. The Patriots prepared for this season's WR situation in many ways, and unfortunately they were not lucky enough for those ways to pan out, but to say that they didn't do anything because "they have six million dollars in unused cap space" is naive and a stupid attempt to criticize them now for stuff that was out of their control or a reasonable assumption then (like that Deion was going to play this season).