Why would one not want to have the anchors simply because there is no term that exactly starts with those two letters?

because the presence of the anchor suggests, at least to me, that there are such terms. a matter of personal preference, i guess.

1. hacker wants to have a page per letter of the alphabet

ah, that was not clear to me. while all of the example data did start with P, i saw no indication that other starting letters have their own page. in fact, i read this part (from OP):

I can cut down the amount of vertical scrolling through a long list, by making them "Pa", "Pe", "Pi", "Po", "Pu", for each letter of the alphabet ($letter . [aeiou];)

as saying that all terms are on one page.

in general, i question the use of vowels for these transitions. for example, between a and e are b, c, and d, which are not very common as second letters, except for c when proceeded by s or for words that begin with vowels (which don't often have another vowel in the second position..). similar distribution issues lie between i and o.