Isn't it simple? CB is like most units, the starters are in place but adding a quality 1st rounder could move the unit from good to very good...
Where would this "quality 1st rounder" play or see the field considering that we have at least 4 solid starters at the moment for 2 positions and the wildcard that is Ras-I-Dowling which would make it 5 starters? I don't think it's quite as simple as you may be making it out to be.
Belichick paid Arrington handsomely at 4 million AAV when all of the rest of the league's top CB's were bringing in 5 million AAV (and even less in some cases). That tells me that he sees him not only as a slot guy, but a player who will take over reps outside--whether we like it or not, so we have to include him too.
Impossible I know but imagine if the Patriots actually have too many CBs? Kind of like having too much pitching.
I suppose if you believe that we're all set at the positions of WR and DL for now and the immediate future AND you're good with not trading down to acquire an additional 2nd and 4th rounder, then this would be the answer for you, but keep in mind that we have pretty much no one at either position for the future at the moment.
The following DL players are all due to hit FA at the end of the season: Ninkovich, Love, Deaderick, Cunningham, and Pryor.
The 2 that wouldn't be due for another year are Wilfork and Kelly.
Either way you see it, we need some youth for the future--and the position itself already needed an upgrade in talent and the ability to rush the passer before even taking this into account as it is.
We all know the lack of talent and depth at WR too, where we'd like to add a speedster who has some physical traits and size. The 1st/2nd round of the NFL draft is pretty much the only place you're going to be able to get that kind of guy for a reasonable price...so that is the debate to adding yet another high pick CB, the specific prioritization of the needs. It's also why I'd be just fine with adding someone in the middle rounds and/or a vet for this year, and waiting until next year to address CB with another 1st/2nd round pick.
We'll have to see what Bill Belichick thinks though, as he's the only one who's opinion counts