Wandering plague isnt a bad talent, its iirc, a 1.94% increase per point, so it isn't bad at all, but for a main tanking Dk there is better talents to get. Also wp has internal cd
Do a wws for Your run, and You'll see that wp will do about 2 maybe 3% of the damage at the end of a heroic run, And that;s with dps Dk, that has almost 30% crit, You have 8%...

Also about desecration:
Just a bit of an aside here, Desecration is an awful talent for a tank (and a pretty bad talent for any PvE situation). The fact is, due to Epidemic, you just aren't Plague Striking often enough to refresh the 12sec Desecration (and if you are, you're nerfing your damage much more than the 5% would provide). So, it ends up being an increase in DPS by about 3.33%, which is pretty low for the steep cost of 5 talent points (Blizzard has stated that for DPS talents, the "rule of thumb" is about 1% DPS increase per talent point. Plus, if you're DPSing with the Glyph of Scourge Strike, Desecration has even more downtime, sometimes drastically so. Desecration is a PvP talent, through and through.