They always used to say the longest word was "antiestablishmentarianism".
Actually, I don't know what happened to all of my posting but I did say,
Anyway, it's all simply pseudointellectualism,
a very common flaw among antidisestablismentarianists.
I guess I wasn't supposed to add the "dis."
(Famous last words of Baghdad Bob.)
O, and the distinction of course, is that pneumonoultramicroscopicsilicovolcanoconiosis is terminology (or jargon), where as antiestablishmentarianism is, supposedly, regular English.
Some people don't like including jargon because, for instance, organic chemistry names can become endless. For instance, you can specify each quirk of a nucleic acid, adding a complex prefix to each step. (Although, by common American conventions, hyphens and even commas are typically added in the middle of words to make them more readable!)
Picture relacing each of the 4 Sodiums in tetrasodium ethylenediaminetetraacetate (a common shampoo ingredient) with a different base. It gets real ugly, real quick.