If you look, those leaves have probably been replaced by newer ones. I see this on my cherry tomato plants especially -- little shoots emerge between the main stem and a branch, and then the old branch dies.

I just pick the brown leaves off. Not for any real reason, but because they make spotting cherry tomatoes more difficult.

mudster, I know what you are talking about, as that happens to my cherry toms too.
But no, the leaves I am talking about are NOT brown--they are just darkened, almost a soft black/green. Almost like green tea leaves, which retain their green color when dried, but are still green. And so very thin at this point, I can only describe as "crispy". The brown leaves on the cherries are "thick" and brown.

Sometimes it happens when the plant dries out; if the water evaporates quickly, the tissues in the leaf dessicate before the rest of the plant can recover any chlorophyll from them. Windburn does the same thing, by drying out the leaf rapidly.

--Alison