Generally agree, but should be supplemented with influence of hand-written math. > Xi -- seems like a good one! This one is fine to typeset, but is slow to wr

Generally agree, but should be supplemented with influence of hand-written math. > Xi -- seems like a good one! This one is fine to typeset, but is slow to write by hand because it has three disconnected strokes. (You have to pick up your pen twice.) People don't want to use it to do actual hand calculations, and so rarely use it in papers (both because they aren't used to it and, perhaps, because they know their readers won't want to write it by hand). > Pi -- good one, I have seen this used for sets. In context there is rarely any confusion with product notation. I think the rarity is indeed because of risk of confusion with product notation. Note that capital sigma is also very rarely used (15th), and my guess is that many of its appearances come from people mistakenly using it for a summation rather than \sum, e.g., \Sigma_{n=1}^N. When typeset, the size of the summation symbol is pretty obviously bigger than the variable Sigma, but in handwriting it's not as clear. It's true that subscript/superscript notation for the variable being summed over (and that variable's limits) tends to disambiguate, but very often in handwritten math people just write a quick squiggle for the summation sign without bothering to write down the summed variable because it's obvious from context. So overall, I think the rarely used Greek letters are rarely used for pretty good reasons! I'd say Upsilon is the most unfairly ignored. It's rendered like a Y in some fonts, but not the one that TeX uses for math, and it's easy to draw quite distinctly by hand.