Theory: Many real-life railroad yards did not have separate yard leads, so they don't belong on model yards. Anyone who uses yard leads is simply following what other modelers have done like a bunch of brainless lemmings.
Practical experience: After one has helped build layouts, design layouts, and operated on many layouts, one will observe that most model railroaders run much larger numbers of trains in a given period of time through a given physical plant than would the real-life railroad. Yard leads are thus a concession to this density of traffic, necessary to keep these high levels of traffic flowing through yards and onto our always-too-short main lines.
The presence of yard leads on many successful layouts is an indication of their utility, not a case of mindless lock-step copying.
Verdict: Except for very low-density one- or two-train-per-day branch lines and terminal switching layouts, yard leads are often worth considering to ease traffic flow and allow more operators to have more fun on a given physical plant in the model -- especially given our short main line runs.
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Hey, if you want to model bollixed-up yards and have your operators standing around twiddling their thumbs, more power to ya'. I'd rather have the traffic flow -- call me crazy!
Don't know what constitutes a yard lead?
A yard lead extends the opposite direction from a yard ladder, allowing a switcher to work without fouling the main. Note the crossover that allows trains to enter/exit the yard from the main.
The yard lead may connect back into the main at the far end (to the left in this sketch) to allow an additional path into and out of the yard when things are congested, but that is optional.
Craig Bisgeier's site explains in more detail.
