After reading the other answers, there are a few questions to adress here, but I must say in advance I'm an inexperienced road driver (2 1/2 yr on a road bike, with about 2k-3k per annum). There is no correct answer, but it depends on the situation. Mayor factors affecting the situation are speed, steepness, wetness, curviness and your positioning on the bike.
On flat, straight and dry patches with your derrier on the saddle there is no harm also giving the back brake, next to the front brake, a tighter clinch. If the rear really starts skidding you won't loose control.
If your going downhill on a dry part with curves on a road bike, I can't really move my ass behind the saddle because I tend to drive wider curves, therefore I only use the back brake a little bit, so that it won't start skidding. If your back wheel starts skidding while going 60 downhill in a curve, you're lost...
When watching rookie mountainbikers going down hill, they tend to hang over the handlebars. If they then apply strong force on the front wheel they may really tend to topple over the front. On the road bike you are alot lower and therefore don't go over the front quite as quickly. I only once tested how much force it would take to lift the back wheel with a regular braking position: I was going about 35-40 and hit only the front break really hard on a roadbike in normal undergrip position. I could feel the rear wheel slowly lifting, but could reduce the breaking power long before I started to topple. The speed reduction was massiv.
Now the more interessing situation: I was road biking downhill on an up to 20% step road, with some wet patches. Without doing anything you excelerate up to 30-40 in an blink of an eye. Because I was hitting too high speeds on a wet and curvy patch I cliched the front brake harder, but noticed that it wasn't really slowing me down, therefore I panicly pulled the back break. It started skidding, and I nearly lost control, lost my concenration and had to let go of both breaks shortly and so on... Leason learned: On difficult and fast patches only use your back brake for minimal speed reduction, everything else endangered me.
In all cases I never noticed the front wheel loosing grip, which would certainly involve a crash. But I think this is only true for road bikes, strong disk brakes may well be able to block the front wheel so that it starts to skid.
To get back to your question: If you are skilled and don't panic and are able to dose the front brake quite strongly and the back brake really softly you will stop faster than only using the front brake. But most people are not able to "feel" the grip of both wheel and adjust both clinshing presures simulatiously, resulting in backwheel skidding or to weak breaking on the front wheel or some other really weird shit. Therefore I understand Sheldon's advice, even if I don't know his rational.
Conclusion: Learn your skills on safe parts, but don't risk doing experiments on dangerous slops. Only pulling on brake while learning also helps you understand how much only the front or back brake can recude your speed.
EDIT: On normal boring straight flat were I have to slow down because of traffic or such I use both to reduce wear on the breaks and rims.