The highest LB drafted in the BB era was Ryan Claridge (5th).
Undersized DEs are used by BB to play OLB, not ILB. In Bruschi's case, he was initially used as a situational edge rusher, but was too small and slow to be effective in that role. Years later, Bruschi developed into a ILB.
It will likely take at least 3 years for a rookie LB to learn BB's system. Therefore, since the Pats are playing for the present, it's unrealistic to expect a rookie to come in and start in 2007. Moreover, this doesn't look like a strong draft.
Why didn't BB draft LBs on day one? Because none of them were worth drafting when the Pats were on the clock. Therefore, FA is the only way to go.
Why Briggs?
Here's a list...
2007 NFL FA LBs
Outside of Briggs, I don't see any Vrabels or Tedys on that list. Maybe Thomas. But he'll turn 30 next year and will also have his suitors. Better to overspend on a great LB then a solid one.