It's incredible how poorly some people understand the salary cap.
The Patriots this year signed three young, premier free agents -- Talib, Amendola and Vollmer. They spent pretty good money on each. In the last few years they've extended Gronk, Hernandez, Mayo, Wilfork, Mankins, Gostkowski, and Slater, all to premium contracts. They re-signed their best player this winter, adding three years to Brady's deal.
Teams like Miami that do what you propose, signing those "2-3 impact guys" that will "put them over the top," could only do so after they let their own impact players walk -- in Miami's case, they let go of their their two top offensive players, Jake Long and Reggie Bush, and three of their best defensive players in Sean Smith, Karlos Dansby and Kevin Burnett.
If you want the Patriots to chase players like Mike Wallace or, I don't know, Cliff Avril, you have to be prepared for what that means -- losing players like Vollmer or Talib. Good teams cannot retain all of their own good players and then also add premium free agents. It's simply not possible mathematically. In the best case scenario, a team keeps its own key players and then has to scrape and claw to fill in the gaps with the draft and street free agents.
The Patriots do a great job of keeping all their best young players. It's also good for the clubhouse, where guys see that they will be rewarded before some outsider if they do things right.
Anyway, it's just amazing to me that people don't realize that under the cap, you can't have your cake and eat it, too. You can either have Mike Wallace and lose Vollmer and four other guys, or you can keep all the good players you spent years developing, players you know can keep this team in contention, and then try to spend what little capital's left making (sometimes incremental) improvements. Upgrading the kickoff return from McCourty to Leon Washington for a million bucks is a great example of the kind of move winning teams need to make. Getting Adrian Wilson to cover tight ends instead of Brandon Spikes is another obvious improvement -- he cost one-fifth what Ed Reed cost, is two years younger and was in the Pro Bowl just two years ago. Tommy Kelly on your pass rush instead of Kyle Love -- you preferred Desmond Bryant for $34 million or whatever? Donald Jones as your fifth wideout instead of Deion Branch is probably a net plus. How many teams have a backup swing tackle as good as Will Svitek, a guy who started at left tackle on a playoff team?
All of these things are smart, economical moves. There's probably one or two more moves coming, too -- an Abraham or a Freeney. They're going to spend right to the edge of the cap, and what they don't spend will go to extensions. Exactly what else would you have them do?