Very odd, I used some of these rare symbols a lot and thought they were more frequently used:
\varrho (ϱ) - for density
\varsigma (ς) - I don't remember for what, but the regular sigma was already used
\vartheta (ϑ) - In spherical coordinates
I think some of these are because these letters look closer to handwriting on a blackboard (the way it was written at my university at least).
I've also used a different kappa, because the regular kappa looks to much like a k, and the variant doesn't look like a k at all. Also, I've sometimes used a variant latin letter v, because the regular v in latex looks to much like a u.
\varrho (ϱ) - for density \varsigma (ς) - I don't remember for what, but the regular sigma was already used \vartheta (ϑ) - In spherical coordinates
I think some of these are because these letters look closer to handwriting on a blackboard (the way it was written at my university at least).
I've also used a different kappa, because the regular kappa looks to much like a k, and the variant doesn't look like a k at all. Also, I've sometimes used a variant latin letter v, because the regular v in latex looks to much like a u.