fiery got buffed since WOLTK release....at least that's what i was told by a lifer 19 rogues, that used sport duel +15agi. we know Blizz does changes that never make the patch notes.
recently, to see +15agi being dropped in the last 4 months and players going to fiery, amazes me. i also ask rogues i have seen over my play period of 2 years: "why fiery?" and overall they say it does more damage now.
i started playing 2 years ago i built my econo 19 twink rogue with duel fiery that was all i could afford. all the advice was go duel +15agi. all the pro build sheets were duel +15agi. i NEVER.......NEVER have seen fiery recommendations like the ones rolling now. nor have i ever seen the praise for the use of fiery for twinking until now. all the talk of 19 rogues was SF/CB/TB/WB/AB, etc with +15agi.
when i built a 29 rogue, i went fiery till i could afford +15, at that time, once i had dual +15 he seemed to do better. but now it appears blizz has done some secret changes on fiery and may have to look at that bracket too.
in fact, in the past prior to WOTLK on RUIN BG you would run into self-designated pro rogues who would flame players with: fiery, LS, crusader, +wpn dmg, etc. with comments of LOL, noobness, etc.