But with the sheer volume of picks we have going into the draft, trading for a vet is a possibility I haven't considered much yet, but it could be a possibility.
I consider "sheer volume of picks" to be a bit of a misnomer. Prior to awarding of compensation picks and/or a trade for Cassel, New England has 8 picks. We are hopeful there will be one or more picks from a Cassel trade, and the compensation pick projections will be rewarded with at least one mid-round and one late round pick, but that's not the same as knowing NE will have 10+ picks on hand.
It's also worth noting the three successful examples you cited involved giving up two Early picks, one Mid-Round pick, and one Late pick. "If" this draft is as strong as some have conjectured (and I'm leaning in that direction), then yielding one or more of the early to mid-round NE picks subtracts significantly from the team's ability to mine this draft class.
The two unsuccessful examples where both Mid-Round. Both involved trading for someone to
fill a hole in the team - 'desperation' trades as it were. Your Safety example for this exercise would be a trade along similar lines - you aren't taking advantage of another team's predicament to upgrade a position, you are trying to fill a hole in the team.
Further, I believe Free Agency rules will change for 2010 if the CBA is not renewed in some fashion: playoff teams will have restrictions on their ability to sign free agents. If that does occur, New England is as sure a bet as any team to find themselves handcuffed by the new rules and the draft will become even more critical for team building. Excess 2009 picks rolled forward to 2010 or further (the way the Pats have converted a 2007 late third into a 2009 mid-second) have to be carefully weighed against a trade this season.
When I consider all the implications, I'm reluctant to consider a trade unless it was for an upgrade (as opposed to a "patch" trade), for example; if Dallas were to continue their effort to follow Oakland's leadership in dysfunction and decided to remove the "cancer" of Jason Witten - that would be a trade worth discussing. Young, productive, team player being run out of town to appease the wart on the owner's backside - it's a no-brainer upgrade at TE (price being another factor). I'm sure someone wants to use Sean Jones in Cleveburgh as an example of an upgrade, perhaps, but in this case New England would be the team initiating the trade talks, and would now be approaching the trade from a position of "need." Now wouldn't everyone's favorite HC of the CB make that painful?