There is an additional angle to Pete Carroll's "worst play" call in the Super Bowl. The Seahawks had stopped the clock at 26 seconds and they potentially had 3 more downs to get the ball into the endzone. However, they only had one more remaining timeout at this point, so Carroll could only stop the clock after one more unsuccessful running play (assuming the play stopped in-bounds). There had already been one running play (on first down) that one did not reach the endzone - this was Lynch running from the 5 yard line, where he gained nearly 4 and 1/2 yards to the be just 1/2 yard from the endzone. That play required the use of Seattle's next to last timeout at the 26 second mark. So they could have run the ball only once more WITH the guaranteed ability to stop the clock. If there had been no gain on a 2nd down running play, let's say they used that last timeout. Then, if there had been another no gain running play on 3rd down, what would happen next? The answer is that the Super Bowl would most likely be over at this point, and no 3rd run attempt by Seatle on 4th down would even be possible (unless the Patroits had stopped the clock, and why would they with the lead and time expiring?).
I believe that Pete Carroll had this very issue in mind and he decided that one passing attempt should be made in this final series of plays, because an incomplete pass would not require a timeout, meaning that their last timeout could be saved for use between 3rd and 4th down (assuming a run on 3rd down did not score, or reach the sideline). This scenario (at least in theory) would preserve all 3 of Seatle's potential chances at the endzone - while there was no easy way that the Seahawks could have made 3 more running attempts at the endzone (this would require one additional timeout). The non-easy way would have been for the Seahawks to somehow manage to squeeze-in a 3rd and final run attempt by using a Philadephia-like "hurry-up offense" approach after any failed running attempt on 3rd down. Carroll's other main concern was to leave Tom Brady with little or no time to work another miracle comeback for New England in the final few seconds.
On the other hand, calling for a sudden gear-shift into "hurry-up offense" mode at such an ultra-tense moment in the final seconds of a Super Bowl might have been asking quite a lot of Seatle's 3rd year QB, and even Seatle's very best "hurry-up" performance might have still resulted in cutting it very close to losing their 4th down possibility and the game, since Seattle had been fairly slow between plays for most of the game. As I recall, Seatle's sometimes languid style between plays had even been the cause of burning their first timeout of the 2nd half (i.e. a Seattle timeout used earlier in the half just to avoid a delay of game penalty). I think that this effective limit on the Seahawks of only 2 more running plays is the reason that Pete Carroll used the slightly odd terminology about "wasting a down" in his post-game comments, with a pass that would either score the winning TD or stop the clock with an incompletion. But he failed to mention another possibilty on that already infamous 2nd down pass play, a show-stopping interception.