The primary argument for going to double-butted from straight-gauge spokes is spoke fatigue. Now fatigue is something that develops, in a reasonably well built wheel, after 10-15 thousand miles or more. Who's ever going to ride a wheel on the track that much? On the track, excessive spoke crosses and double butted spokes both lend to lateral (sideways) flexibility that's comfortable on the road and prolongs the life of the spoke, but is largely irrelevant on the track. You really want not just the best torque transfer, but also the best lateral stiffness so you can attack a banking without being concerned about your wheel flexing and feeling unstable under you. For that reason, I've shifted to building all the track wheels for our team and for others with radial fronts and 3-cross rears -- no 4-cross rears, no crossed fronts -- and with 14-gauge straight gauge spokes (unless someone requests an aero spoke as an alternative).

For road fixie riding, it's a somewhat different ballgame. Compared to a typical road hub, you are far ahead in the game -- you don't have the extreme dishing on one side of the rear wheel and if you have a high-flange hub you have better support and attack angles on the spokes. A straight gauge spoke can last you for a long, long time. But a double-butted spoke will still last a bit longer (25000 miles instead of 20000?) and will feel more flexible and comfortable to ride. The harder you are on your wheels, the more double-butted spokes are appropriate on a road fixie.

Tension-wise, Mavic recommends 950 n-m for pretty much all of their rims. That's pretty low compared to what a good wheel build really needs (it barely is enough to pull all spokes into common tension) and many mechanics bump up to 1100 n-m or higher without problems. Mavic uses rim extrusions that are quite thin and aren't any thicker at the spoke hole; they depend on the ferrules to hold the spoke nipple in place, but the ferrule actually requires a bigger hole in the rim, and the whole thing develops a bit more of a tendency to pull through. Ferrules are not necessary except to deal with very thin-walled rim extrusions. That's why rims like Velocity's, various superb Araya tubular rims (like the R-40 and R-50) and so on do not have ferrules and actually outlast a Mavic. I'm not knocking Mavics -- they are great rims and I love to use them -- but it's just about design compromises. You save a little weight with a ferrule and give up reliability. Buy a heavier Mavic and the problem goes away. Anyway, if you're doing a Velocity Deep-V or Pro-Elite, you can easily do 1400-1450 n-m and not even blink; I've built them quite a bit tighter and never had a problem. You'll never break a spoke with excessive spoke tension -- the rim will fail first. The longitudinal tension on a spoke just isn't even close to what it takes to break it. Only fatigue will break a spoke, and tension too low can allow it to bend back and forth more, leading to workhardening and an earlier break.