Who is Marcus Trufant? He's been hailed by his coaches and is generally a fan favorite. He's quiet, works hard and doesn't miss practice. That second statement might be the cause of the first statement. It's not that we know Trufant is bad, it's just that we have so little evidence that he's good.
Trufant took over the #1 cornerback duties after Ken Lucas left in free agency in 2005. In those two seasons Seattle has been very poor defending against #1 receivers: Ranking 20th in 2005 and 23rd in 2006 as measured by DVOA. But then, for the past two years Seattle has been bad at defending all receivers, besides a couple modest showings against receiving backs and last year's surprising performance against #2 receivers (-7.1 DVOA), Seattle hasn't been in the top 20 at defending against any type of receiver since 2005. It would seem Trufant is not doing his job, but that no one else is either.
If DBs had permanent assignments it wouldn't be hard to assign credit and blame, but the problem is, especially since Seattle plays a ton of zones, that it's hard to know who to credit when something goes well and who to blame when something goes poorly. It's simply a lack of good information. Not only are defensive stats inconclusive--when a DB makes a tackle did it prevent a first down or was it because he blew coverage? Is he tackling his man or is doing mop-up work for someone else who was beat?; Interceptions are always good, but players who attempt interceptions too much can be a huge liability in coverage; etc.--but television coverage is misleading: Unlike the front seven who can be seen from the time of the snap, most of the DB's work occurs silently, in the periphery. The best thing a defender can do in coverage is simply take his man out of the play, prevent the quarterback from attempting to pass to the receiver. If he does that to perfection, he's practically a ghost never to appear onscreen. 
That's why big hitters and interception hawks become stars, but strong coverage players toil in obscurity. Deon Grant knows that. Grant has never been talked about in the same breadth as Ed Reed or Roy Williams, but now he's being paid that way. Grant doesn't have the total package of an Ed Reed or the "upside" of a Roy Williams (just as soon as he learns to defend a pass more than five yards down the field) and some of his contract is certainly market-inflated, but he does have an excellent reputation for deep coverage. That little factoid certainly warms the cockles of John Marshall's heart, and, for us, brings us back to Trufant and the motley bunch we so affectionately call Seattle's secondary.
For a second, instead of assuming that one piece of information precludes the other, that Trufant can't be a good cover corner and yield such terrible results/that the stats can't be accurate when Trufant is such a good cover corner, let's assume both are true. If Trufant is a good cover corner, but Seattle is still weak against #1 receiver and Trufant's numbers still suggest that opposing wide receivers are tallying successful receptions against him, then maybe he's simply being asked to do too much. What do I mean by that? Let me explain.
More than strong performance, Marshall and Mike Holmgren's kind words about Trufant imply confidence. They believe that Trufant is an excellent cover corner. They believe he can effectively lock up his man without assistance. Therefore while Kelly Herndon is given safety help over the top, Trufant is put on an island. It's not that Trufant isn't good, just not good enough. The Hawks are using him like he's Champ Bailey and opposing quarterbacks are picking on him because he's not.
It seems like Seattle has been attempting to fix a broken secondary ever since Shawn Springs left town. Every few years Seattle enters a binge/purge cycle where they throw around cap dollars and draft picks while saying sayonara to the last round of failed free agents and draft picks. Ken Hamlin isn't a poor player, but for the Seahawks, he was deemed a poor fit. Hamlin is a high risk playmaker on a team that already has one in Michael Boulware. Hamlin was asked to play as a true safety, that is, in deep coverage, but never took to the role. Kelly Herndon was brought in with Andre Dyson. He was brought in to play nickel and his combination of big play ability, run stopping and solid short range coverage would have made him well suited for the spot. When Dyson went down he was thrust into the starting lineup, and he was just that, a nickel corner playing as a #2. Essentially Seattle had a nickel playing corner, a strong safety playing free safety and a 46 back playing strong safety. When the pass rush was super effective their were plenty of hands to put on a pick-off display like against the Giants in week 3 or against the Eagles on Monday night in 2005, but when the Qb was given time you had near continuous blown coverage.
Seattle's secondary has a chance to be very good next year. Kelly Jennings looks fit to take over the #2 cornerback duties, the upshot lands us a very capable nickel in Herndon. Grant shores up a huge hole deep, and if he's as excellent as rumored, Boulware should be allowed to free lance closer to the line (his strength). And Trufant will no longer have to be Champ Bailey on a secondary full of misplaced role players. Barring injury, Seattle has finally given Trufant a chance to succeed, and with him our secondary.