I have to disagree with Jim McCoin's observations on frame size and length of seat tube.
fashion in the 20 & 30s was for smaller frames, with some very long seat tubes being used.
This fashion changed and in the late 40s through to 60s when tall frames were the order.
It has now swung the other way, although I believe it is taken to its current extremes to allow frame builders to quote very low weights for frames.
My opinion is that head tube extensions are useful on frames for tall riders as it allows a horizontal top tube to be kept to a reasonable height. They are also useful for those riders who prefer to sit their bars higher (Even as high as the saddle, which is an un naturel thing to do).
My next grass track frame will be built with a slightly extended head tube as I am now of the opinion that I want my bars ever so slightly higher, but I like to have the stem right down in to the steerer tube, putting the fulcrum as close to the head bearings as is possible.
These observation apply to light weights only and are not applicable to roadsters and the like.
Martin Coopland, Scotland, http://www.BatesBicycles.com