For a rider like Wade Simmons, the benefits of lacing the wheel properly (drive side leading spokes heads-in, disc side leading spokes heads-out) would definitely show up in greater ultimate strength and longevity of the wheel. Guys like that push their gear to the absolute limits on a daily basis. He might even notice that it is a bit stiffer under braking loads than wheels laced more traditionally.
For mom, ultimate strength is probably not the greatest concern, but the ability to stay true for long periods of time is. A wheel laced as I've described will stay true better than one laced the traditional, "leading spokes always heads-in" way. Less tension demanded from the spokes by external forces will equate to less stretch, less stress on hub flanges, less bending force on the axle, and a better wheel in general.
It is easier and faster to lace wheels the common way if that's what you're used to. Myself, I build each wheel with a specific purpose in mind, which depends on the weight of the rider, type of brakes (rim or disc), type of hubs (i.e. no radial spoking on hubs that don't have reinforced flanges), and intended usage. As such, I very rarely build two identical wheels, so there would be no speed benefit for me to lace disc wheels the way rim brake wheels are laced.
Personally, I don't care how long it takes me to build wheels. All that matters to me (and customers) is performance and durability. It's also a matter of pride - I won't put my name on something unless it's the best I can make it.
Banzai, good question about how common proper disc wheel lacing is vs. traditional lacing. I checked every disc-brake equipped bike at my shop (Alternatives) today, and found very close to a 50/50 split between disc wheels laced as traditional rim-brake wheels (which SHOULD have leading spokes laced heads-in) and disc wheels laced properly. The machine-built Trek wheels were mostly traditionally-laced, with a few laced properly, more likely due to fluke than anything else. The Rocky Mountain bikes, all with hand-built wheels by WheelTech (Rocky's in-house wheel builders), were more interesting. The builders at WheelTech sign the rims on each completed wheel. I found that spoke routing preference was not only different between builders, but even the same builders built the same wheels differently each time! For example, I was encouraged when I found that the builder who's initials are "MG" built the front disc wheel of a Rocky Flow 2 properly - leading spokes heads-out - but then the rear wheel of a Flow 1 built by the same guy had the leading spokes heads-in on the disc side! I also found that the drive sides sometimes had leading spokes heads-in, other times heads-out.
I can only conclude that the majority of builders don't care about spoke routing at all, and probably don't even know of the advantages of routing them a certain way. In my opinion, they should.