Heh this thread really opened my eyes about the Warrior tank T10 4 pc bonus. At first I thought the 4 pc was great, until I read it only "absorbs" damage. I originally thought it reduced damage by 20% for 10 sec. Silly me! Still, I'm sure the absorb can come in handy in more magic-heavier fights.
Anyway, in a nutshell it's either:
Sexually delicious itemized non-set pieces, where the sum of the stats can make up for the lack of fun set bonuses.
Not as well itemized set pieces where the set bonuses make up for the not-quite-as-good itemization.