Ughhhhh!
Value is made up of many things - need is one of them. If we perceive a need pick, it is a value pick because the need is so great, the player returns the most value.
Its already been established that NEED is part of the Value equation. However, the value is also defined by the tangibles and intangibles of a player. If you've read any of the articles on how BB sets up his draft board, you'd know that need, while a factor, is not the DRIVING factor.
I've given a general outline of what BB has said their draft style is. You can sit there and pretend like its not, but its right from the horses mouth.
Ty Warren was a value pick because he could start and contribute and provide HIGH RETURNS almost immediately. That is VALUE - returns less investment. He filled a need, and that made him valuable (ie need was a significant component of value - need created value)
Warren was a VALUE pick for many reasons, but not for what you stated. Warren was NOT expected to start immediately nor was he expected to provide HIGH RETURNS immediately. Warren was drafted while Bobby Hamilton was still a starter on this team. Many fans and pundits thought Warren was drafted to be a NT. However, this proved to be false when BB went out and got Ted Washington prior to the start of the season, but after the draft.
Warren was a VALUE because he'd have time to learn the position and would not have the pressure or expectations by the team of being an immediate impact. Warren was a VALUE pick because the Pats did not have a need at DE because they already had Seymour and Hamilton.
Warren definitely wasn't a NEED pick because the Pats were returning Hamilton and Seymour. Warren certainly wasn't the BPA at the time.
Warren was certainly the best value because he had the tangibles and intangibles that the Pats were looking for and would have the ability to grow into the position.
Darelle Revis was a NEED pick for the Jets.
Ben Watson was a VALUE pick for the Patriots.
Another way to define a NEED player is someone who has to come in and perform immediately because the team doesn't have anyone better.
A VALUE pick is a pick that the team takes to allow them to develop with the expectations that they will move into the starting line-up given a year or two as an understudy.