According to Jobst Brandt, double taper spokes increase fatigue life by the thinner section stretching to relive the stress in the threads and elbow. However, front wheels usually do not see many spoke failures since there is much less weight on the front wheel than the rear.
A double taper spoke should be weaker than a straight 14 gauge spoke. However, according to Jobst, spokes fail from overstressing at about 3 times the loads that are encountered in normal use. Nearly all spokes fail at the elbow or nipple with a clean break indicating that the spoke has fatigued.
Therefore, a double taper spoke should be more reliable than a straight gauge spoke.