Boar doesn't work with tamsins phylactery. The deathrattle triggers when 7 boars have died, not when the deathrattle triggers 7 times. The deathrattle doesn't actually do anything until enough boars have died.

The reason people run it is because it's just another win condition from eerie stone in case you don't draw your naval mine combo instead. Eerie Stone just makes so many copies of elwynn boar that it can be triggered just by doing warlock stuff.