I hate to take issue with the great man himself, but I think Sheldon's glossary article ( http://www.sheldonbrown.com/gloss_e-f.html ) on extension levers (aka 'suicide levers') is 100% wrong. I'll to address his article point by point...
Sheldon: "In the early 1970s, many people bought bicycles with drop handlebars, for reasons of fashion, even though drop bars did not suit their casual riding style. Given the frame and stem designs commonly available at the time, it was generally impossible to get drop handlebars high enough up to allow a low-intensity rider to reach the drops comfortably."
Many people in the 70s may have purchased drop handlebar bikes for reasons of fashion, but drop handlebars are still produced for touring bikes because they generally offer more alternative hand positions than any other bar (especially when combined with extension levers). This is not a fashion statement - it's an ergonomic necessity for anyone who spends more than a few minutes on a bike every day.
Sheldon: "The problem was worse for many women, whose shorter torso made it hard to reach forward to the drops. Though a taller handlebar stem with less forward reach might be installed, this often did not occur. Also, small hands could not comfortably grasp typical drop-bar brake levers of that time."
Taller handlebar stems were available in the 1970s. Failing to install one is hardly the fault of the bike (or the extension lever). It is the responsibility of the bike shop to sell bikes that fit their customers - this hasn't changed since the 1970s, nor has the willingness of some to sell (and to buy) bikes without ensuring proper fit.
Sheldon: "Dia Compe invented bolt-on extensions that allowed Weinmann-type brake levers to be operated from the tops and middle of the handlebars, making this type of bar bearable for casual cyclists..."
I don't know why Dia Compe invented the extension lever. I do know that it was not merely used by casual cyclists. As a bicycle tourist, I relied on the extension lever to moderate speed from the bar top or (underhand) from the bar top 'corner' (where it starts to curve). The extension levers are great for this. I agree that they are not suited to full-on braking from moderate or fast speeds, but I do not believe they were intended to be so used. If they had been, they would have been engineered differently so that they applied the brake fully.
Sheldon: "The extension lever partially applied the main brake lever, reducing the available lever travel. Not all brands/models suffered from this, but the most common ones did."
I have used various models of extension levers for 30 years. I have never felt that the main brake lever was reduced in travel significantly by any model I've used. The ones Sheldon is talking about reduce travel by, at most, 3/16" - and on every extension-lever-equipped bike I've had, the top of the main brake lever is shorter to take this into account. But even if the earliest ones weren't, it is hardly a problem - cable stretch poses the same problem and it has the same solution: brakes can be adjusted.
Sheldon: "The attachment hardware precluded the use of the top of the brake lever hood as a comfortable riding position."
I have never felt that the hoods were uncomfortable while using extension levers. Yes, they are an extra 'lump', but hands are flexible. The idea that it 'precludes' the hood as a comfortable hand position is going a bit far. Anyway, one of the most common extension lever brakes is streamlined so that it is comfortable.
Sheldon: "They encouraged the practice of riding with the hands on the top, middle section of the bar, which is a position that doesn't give very secure control, especially on bumpy surfaces, because the hands are too close together."
Extension levers "encourage" nothing of the sort! They permit more safety in alternate hand positions. This is essential for the long-distance tourist, and it is something that no other brake system permits - not even interrupter levers.
Sheldon: "The hardware that held the extension levers to the main levers was prone to fall off."
In my 30 years of cycling as an adult, including over 10,000 miles spent touring on a bike with extension levers, I have never yet had an extension lever fall off a bike. If they fall off, they're either missing the lock washer, or they're not properly tightened.
Sheldon: "In the early 21st century, an greatly improved system of "interrupter brake levers " appeared, with all of the advantages and none of the drawbacks of the older extension levers..."
The interrupter lever is not a "greatly improved system". It does not do the same thing at all! It is a brake - it is not an extension. It cannot be used to slow a bike as efficiently or with as much control as an extension lever. I own a bike with an interrupter lever fitted and I own two bikes with extension levers. As a touring cyclist, I would never install an interrupter lever on a touring bike, because it can only really be used as a full-on brake, and only from one position - the top of the handlebars. Extension levers can be used from two positions, allowing speed reduction (not full-on braking) from the entire length of the bar top from hood to hood. No modern braking system allows this.
I feel Sheldon has misrepresented the much-maligned extension lever in his article. Extension levers are hardly the ridiculous and deadly 1970s-bell-bottomesque folly that Sheldon Brown makes them out to be. I think they are unfairly judged by some. For a while, before the introduction of the interrupter lever, they were the only way to slow a bike from the bar tops. I mean which is better - some braking ability from alternate positions, or none at all?
I think extension levers still have a place in some bicycle loadouts - and not just as a quaint V&C anachronism.
But what do you think?