Spoke strength in tension is not really a factor in a wheel. Yes, butted spokes are weaker in pure tension. But spokes rarely fail in tension and when they do, it is usually from defects or damage. Spokes fail far more often from fatigue and the "neck" at the hub flange and less often through the threads (usually from poor alignment with the rim nipple seat). Butted spokes absorb road shock and simply stretch, sparing those high stress locations from the biggest loads. (Easier on both rims - less likely to crack at the spoke holes and hub flanges also.)

All the wheels I have ridden for very long distances have had double butted spokes. Most of the wheels I've owned that have cracked nipple seats on the rim have had 14g (2.0) straight spokes. I will use 14g straight for right rear if I am going 14-15 double butted for the rest. Yes, triple butted or 13-14 would be better but these seem to work and I build few wheels that heavy. The vast majority of my wheels are 14-16, 15-17 and the like (2.0-1.6, 1.8-1.5) and 14-15 right rear (2.0-1.8).

My wheels almost always go until the rims fail; impact or crash damage or brake wear. For the latter I often rebuild with same spokes and new rim if I can match ERDs.