There was a time in the NFL when an offensive lineman was just an offensive lineman. Sure, guys usually played primarily one position on the line, but there wasn't near the level of specialization, mainly at the tackle position, that you see today. That all started to change when guys like "Dr. Doom" Robert Brazile and Lawrence Taylor came around whose sole mission was to give QB's nightmaress. The advent of the pass rush specialist led to the Left Tackle becoming one of the most important, and highest paid, positions on the team.
Now, think about all the really good TE's in the league right now.
Guys like Dallas Clark, Jason Witten, Tony Gonzales, Antonio Gates, and Vernon Davis put up receiving numbers that some teams would be happy with from their #1 WR. Teams just aren't trotting out Mark Breuner type blocking TE's for a majority of their snaps anymore. That's not to say that there isn't a place for guys like that. You still need that "extra offensive lineman" in many situations, and they are crucial in some situations. Still, just about every team in the league wants a TE who can get down the field and make plays as their every down guy.
LBs aren't good enough in coverage for most of the pass catching TE's. If you bring on a nickel corner teams will exploit you in the run game, or use the TE's size advantage over the nickel CB. So, teams need help from the Safeties over the top or for a Safety to match up in man.
It's not just about TE's though. Offenses today will find a way to exploit your weakness. Teams can't just throw out a box safety who struggles in coverage. Teams will find a way to exploit him, no matter how hard you try to hide him. See: Pollard, Bernard.
Getting to the QB is still the best way to defend the pass. End of Story. However, even the greatest defensive lines aren't going to get to the QB more than about 1/4 of the time (unless it's a QB with great hair who falls to the ground in the fetal position immediately following the snap). Even if the defensive line gets great pressure on every play many NFL QB's will still make you pay if you can't cover, and that doesn't just apply to your corners anymore.
Finding the ideal Safety isn't easy to do. A guy who excels in coverage is likely to end up at corner, and a guy with good size is likely to end up at LB. Finding a safety who's equally as comfortable in coverage as he is helping against the run is a tall task. There's a saying that a squeaky wheel gets the grease. Well in this case the position making the most money will get the better players. As the relative importance, and pay, of the position goes up the more elite athletes will focus on it early in their careers as opposed to the Peter principle approach we have now where guys play corner until they reach a level where they can't.
When Eric Weddle signed his monster contract before the 2011 season many people though it was crazy to pay that much for a safety. Troy Polamalu just signed one of those "big for a Safety" contracts too. Ed Reed will probably get a large deal after the 2011 season. Eric Berry received a huge contract as a rookie recently. That had a lot to do do with where he was drafted, but the fact that safeties like Berry(and Sean Taylor before him) were taken so highly gives a hint to the changing importance of the safety position. All of these guys were considered special, but every trend in the NFL has a counter trend that follows it. Brazille and Taylor helped create the franchise LT. Safeties might not ever become as important as a "franchise" Left Tackle, a shutdown corner, or a pass rushing Defensive End/OLB, but their days as almost an afterthought are over. TE's who can play like WR's combined with the sophistication in today's passing game will force teams to seek out guys who can still support the run, but are equally as comfortable in coverage. They will be willing to pay a little more for them too.
(I've been meaning to update this ever since Weddle signed his deal, but Doug Farrar's article on The Shutdown Corner and my desire to not do any real work today finally got me around to it)