I'm one of those who hang out outside Orgrimar and duel for hours.
Lately I've been seen more people gem'ing for spell penetration on their gear. The first guy i saw was a DK (I PvP with a DK too, but I gem for strength /shrug?), full furious, every single gem slot with spell penetration gems in them. Fist thing i said to myself was "LOL, Noob!" but then i saw had a 2200+ rating and some kind of PvP ****** like "Duelist" or something so he must be doing something right. Spell penetration on a DK was odd (at least they do spell damage), but then later on i saw a FERAL cat druid, fully PvP geared as well with spell penetration gems and he had his nice arena rating too.
So what am I missing here? What does spell penetration do that's worth changing all my strength gems?