The Pi symbol has a conventional meaning of course -- Pi is to products as Sigma is to sums. So, I'm sure it is avoided as a variable name for the reason that Sigma usually is.
When I used to teach LaTeX classes, most of my students were math department secretaries, so I always made a point of explaining when someone would want \Pi versus \prod or \Sigma vs \sum.
Of course, being the callow youth that I was back then, I also tended to pronounce the names of the letters like a classicist so ψ is not pronounced "sigh" but rather "p-see", and similar ξ is not "zee" but "k-see." A childhood spent surrounded by people with Slavic surnames had my mouth well-trained to manage odd consonant clusters at the beginnings of words.
Back in physics grad school it surprised everyone else that I not only pronounced 'ξ' as "ksi" rather than "cascade" or "squiggle" but, more amazingly, could actually write it like ξ rather than a squiggle... I still don't really get why that's impressive.
We had one prof who wrote these as ς with one, two, or even three squiggles below. I never figured out what letters they were supposed to be (zeta, sigma, xi?), but read them as something translating to "zig-zag".
ς and ζ look similar to their handwritten Roman equivalents.
ς is end sigma, the letter σ (S) when written at the end of the word.
ζ is zi (Z), it's not unusual to write a lower case Roman z with a similar tail. My mum was taught to do this, which confused me when I was learning to read.
Yeah. Although -- I should note that the article did mention it as well, just toward the end. I was just a lazy reader on my first skim through and missed it.
I bet upper case theta is too close to lower case theta, and nobody wants to put their new variable so close to one with such a strong implicit meaning.
