Gives Pats more flexibility in their defense. They've been going more 4-3 to date, with some 3-4 mixed in, along with sub packages. I think it's safe to say we won't be seeing Seau too much, if any, in 4-3 packages; not enough speed there. Instead, he'll probably be part of a 3-4 package that'll look like this:
LE - Warren/Sands/Brace
NT - Wilfork/Pryor
RE - Green/Wright
LOLB - Thomas/Burgess
SILB - Mayo/Seau
WILB - Guyton
ROLB - Banta-Cain/Woods
The 4-3 package will continue to look like this:
LE - Warren/Burgess
LDT - Wilfork/Sands/Brace
RDT - Wright/Pryor
RE - Green/Banta-Cain
SLB - Thomas
MLB - Mayo
WLB - Guyton/Woods
The sub/rush package will continue to look like this:
LE - Burgess
LDT - Warren
RDT - Green
RE - Banta-Cain
LB - Thomas
LB - Mayo/Guyton
Not included are Ninkovich and Alexander, as they haven't really seen time on defense thus far.
So, I think we're basically going to see Seau as a Ted Johnson run-stopping type - kind of the same reason Sands was brought in. He'll certainly help out in the goal line/heavy packages and on running downs when TBC/Woods/Guyton/Burgess are off the field.