Pretty much... never.
BV is useful for a second set for trash, heroics or light hitting mobs (think Loatheb). Other than that, avoidance and health are better for main tanking.
im not in a progressive guild we are currently just starting 10 mans.
at what point should i start working on getting block value on items instead of avoidance or health?
846 damage blocked
havent had problems with threat *dodge and hit capped*
could replace helm and boots for either more health or more avoidance
I think a more in-depth explanation is in order at least to shed some light on why. There is a point to which you can stack block value and rating to get yourself some useful overall mitigation.
We have unfortunately not reached this point yet. If 4k block value were attainable at the moment, I would stack the hell out of block value and rating, along with critical block we would be removing a whole lot of damage from the attack table. Even possibly 3k block value may be worth it.
What you never want to do, is sacrifice "better stats" for niche stats like block value and block rating. It is fine to have a piece with block rating and value on it as long as you're not wearing it over a piece of gear that is vastly superior. If you find yourself gearing in such a way where things are not working out, you need to reallocate some ilvls somewhere.
We may see 3k buffed block value sometime in the future with epic str gems, I believe if I replace all my stamina gems, along with full block value gear, I can more than likely hit 2k+ at the moment. Though this would come at the expense of more health than I would like. Icecrown is probably the only patch where we may be able to hit such inflated block value #s.
Problem is, blizzard is already watching this and if we do get sets of gear with huge block value #s and critical block becomes moderately useful, where we block say 9k of a 25k hit(probably a good bet for a 25man boss in IC that has normal melee), we will get ourselves a hotfix and again be gearing for effective health.