You can't be great at all positions at all times. We can't start a first round pick at all 22 spots and special teams.
So realizing that, you need to prioritize and define what kind of value you can afford to lock up at each position.
In a 3-4 defense the success or failure of the defense against the pass falls on the ability to do two things. First to provide for a dependable pocket colapsing (your front 3 guys) and second to have OLBs that can rush the passer with dependable success. We addressed most of those issues in last years draft and FA, but still had one glaring problem. We didn't have a second OLB on the roster that could be expected to provide a credible pass rushing threat and still hold up against the run. Without that, teams were free to always account for Ware as a rusher on every play. They shifted the LT directly at Ware and didn't allocate a lineman to the opposite OLB. They felt, accurately, that the back could adequately block whomever was rushing from that side, outside of the DE. Because of that, Ware was limited in reaching his full ability and our pass defense was forced to rely on excellent coverage. Long pass plays (a la Santana Moss) don't happen when you have a reliable pass rush. Therefore, OLB was a need position.
Second, there is an issue of draft position value. All teams, and coaches, value different positions according to their own standard. BP clearly places a strong value on interior lineman compared to tackles. While this might seem in direct conflict with the rest of the league, it makes sense in the same way that using the 3-4 makes sense when the rest of the league is playing a 4-3. The cost of a tackle is much higher than the cost of an interior lineman. Other teams might have superior tackles and substandard gaurds and centers. In this scheme they will likely need to account for any pass rush by either allocating a back to block any interior rushers or forcing the line to squeeze the pocket inwards to reduce space. This method suffers from a few flaws. It creates a lot of traffic in the interior of the line and it places a back as an important blocking element to a much much larger lineman. On the other hand, placing the emphasis on the interior allows you to draft better players for less money (overall across the line) while instead having to account for edge rushers. BP uses TEs to account for edge rushers thus creating a more favorable match up. Much larger TEs (compared to backs) blocking relatively small DEs (compared to DTs). It also puts the potential weapon (either the back or TE) closer to the line of scrimage and thus closer to becoming a real outlet in the pass play. It does have some problems. If both tackles are weak then you end up using two TEs where on the interior you could have used just one back. However, BP lucked into having a very good LT already on roster. It also means that the QB will recieve more presure the further he extends out of the pocket. But when was the last time that Bledsoe went out of the pocket on his own volition? If I had McNabb or Brunell in my backfield, I might want to leave the idea of heading out of the pocket as a possibility by having better and more athletic tackles. Likewise I wouldn't worry as much about the slower interior rush that my QB could probably avoid.
In the end it's about cost. You can only tie up so much money (FA contracts) and resources (high draft picks) into the unit (O-Line). Parcells feels he can find top rate OG and OC talent in the second round but only second rate (or worse) tackle talent in the same position.
As for last year, the plan was foiled for two reasons.
First, Adams went down meaning that both tackles needed TE protection.
Second, keeping both TEs in to help with coverage meant using only two recievers and one back. Those two recievers (Glenn and Keyshawn) were not able to reliable be a threat against the coverage they faced. Glenn was always pulling a safety over the top and Key couldn't seperate from man coverage. This would have been helped if a TE was free to split the coverage, but they were needed to at least chip a blocker at the line after Flozell went down.
Bill address both issues this year. He gets Adams back and he got better depth than we had last year at OT. He also signed a better second reciever that can get seperation from man coverage in case he does have to keep in more TE help. This year will be telling to see if this plan works.
As for drafting according this this plan with our first pick... that would mean either getting a guard, center, or TE. We didn't have pressing need along the inside of the line. KC Joyner breaks down our interior lineman and found that all (including Kyle) had fairly good years. Drafing a player in the first in those positions is reserved for guys that are going to start right away. We just didn't have as pressing a need there. As for TE, it's generally difficult to get a dual purpose TE in the first. Most first round TEs are pass catchers (Todd Heaps) and not blockers. Those guys are generally only average at both skills and fall accordingly to the second or third rounds.