It was added to help the mage class compete with changes to gear. As swing speed kept increasing, so did the odds of interruption, hence the addition of casting focus. With suits the way they are today, it is almost completely necessary to have casting focus against some templates.
BTW, I don't like it either, but it did help to balance the game...
Mages gain more benefit from gear than dexers do, without casting focus. why? because Mages are stronger & more versatile than dexers (they always have been), in every situation except pvm.
The odds of interruption have always been the same, it's the frequency in which they occur that has increased, but since swing-speed is capped at 1.25s, mages can guarantee a mini heal between hits which is ~40-50% of most dexer attacks worth of damage, if a hit should miss, that's ample time for a G-heal which out-heals the damage done in a single hit.
The only thing casting focus did, was destroy mage dueling by adding more RNG to it, couple that with Poison immunity & dueling is dead.
Casting Focus didn't help balance the game. It gives mages an unnecessary edge in pvp, because dexers will always have a chance to miss anyway, and most spells can already be cast between a dexers attacks.
Besides, if it was meant to balance things at all it was an extremely poor attempt by adding a 12-17% chance of "balance". If they truly wanted to balance mages with dexers they'd remove Casting focus from procing from Player-based damage and increase the values Casting Focus spawns on items & scribe bonus to reach 100% for pvm-based damage. That would go much further in terms of balancing pvp and pvm between both dexers & mages.
Also, Casting focus cancels out paralyzes (paralyze/nervestrike) just like protection would but at a "chance" & without casting speed penalties.
