I have to admit a combat log would be great to see a play by play of whats happening.. i too used silence but later removed it for as many zombie spells as i could load along with heal all..its alot going to on to really visually see .. i do see my pet "casting" or using it but what its silencing is unknown.

here's a good way to check:

Mapworks boss General Gnash summons several (insanely overpowered) mage minions when you fight him.
silence prevents mobs from using spells for X seconds, so if those guys suddenly try to melee you - it works.
however, I think it does not work on the boss itself, because some bosses are completely helpless without spells and the game doesn't allow that to happen.

Yet another useless spell in my opinion. Most of the time when I see my pet casts it on enemies they keep using all their skills.