I use to use a crit Fiora build a long time ago. May have to bring it back out and give it a try. Lifesteal is only good to a point. Any statistic in excess is genreally less effective then a semi balanced approach even on champions with innate synergy to a perticular stat.
With her high innate AD from her passive W, and her AS buff on her E, crit could be very efficient 3rd leg so to speak. It's not mathematicaly speaking a bad idea to give it a try.
[Edit] biggest reason most avoid it is because the strength of her ulti is apply on hit effects, and last I checked it will not apply crits.