assassins already have enough fun toys
Yep, this is why I love playing assassin. I don't need to wait for my focus ability to melt my enemies' faces. I can do it in
every fight.
IMO, the focus ability might be worthwhile if it granted stealth that was immune to Perceived. Bring Cole along, and half the party (the half that's responsible for the majority of the dps) has guaranteed crits from stealth. Slap down a Mark of Death, assassinquisitor and Cole chug some Mighty Offense, Cole activates CoS, and go to town.
Unfortunately, most bosses break stealth, and that amount of spike is complete overkill for anything less than a boss. But, if it granted unpoppable stealth...