Assuming we're using vinegar as our acid, it has a pH of ~2.38 off the shelf. Why does most every tek I read instruct the chemist to mix the vinegar with water? Is 2.4 pH too low? If my calculations are correct, in most cases the amount of water called for only raises the pH marginally. Is it just so that we don't have to waste so much vinegar?

It depend wich AB you do

in Wet AB it's mostly (i believe) for consistency, liquid is easier of use than a thick soup in a bottle.
Acid is not mandatory, it's here principally to help breaking plant cell (to yield more). You can do STB (straight to base) without use of any acid, Let your base break cell longer, and job done.
Sometimes you yield more with an AB than STB, and sometimes the contrary.

Thank you for your reply. I am aware of STB teks, in this case I am asking specifically/only about A/B (and A/B hybrid) teks. If it is for consistency as you suggest, why not just add more vinegar instead?

Vinegar is acetic acid diluted in water anyways. Yes you could just use a bunch of vinegar instead of adding more water. The downside would be that you'd spend more money on vinegar (though its cheap so its not so bad), it would smell stronger (you might not like the excessive smell), and later on you'll have to add more base to neutralize it (not a problem if its lye since its so strong but if you're gonna use another base you might need too much )