On paper, it looks like the wrath card is an amazing threat boost for just one slot since we won't have all that much +crit in the first place. Ignoring the shard of contempt for now, if you're more interested in average threat than the on-use snap threat of GAB, why isn't this card more popular among warrior tanks?
Because crit is not great as a threat stat for prot warriors. An equal-level trinket with Hit, Expertise, Armor Penetration or Block Value will pretty much always be better when gearing for threat.
That said, and as you pointed out, the Wrath trinket does provide its highest crit increase when used by people with low crit rate. For that reason it's probably the single best items for us wierdo Shield Slam crit addicts running around with 1k SBV and 15% crit rate. I have one and only use it for my "1H+S grinding" set.