The reason I take it anyways is because I value Initiative (the higher the challenge level of the table, the more important it is), Dex saves, dex skills, and the occasional extra Dex-based attack at low levels, and because the +1 Dex from Moderately Armored makes it convenient. It's kinda similar to the reason why I don't like to dump Dexterity even on Strength builds; the stat is good even if you're not getting AC from it. You can easily drop it from 16 to 14 if you like (as I note in "variants" for the Iron Wizard), and spend the points somewhere else. But extra points in Dexterity are by no means wasted IMHO; it's not like investing in, say, Strength instead would have gotten you an AC bonus either.
Contrast this possible statline for the Hobgoblin Iron Wizard:
With this one:
Or this one:
All of those have the same point buy cost. #3 will make you extra vulnerable in levels 1-3 (with a weaker resourceless attack and AC). #2 is basically just exchanging +2 Dex for +2 Wis. All are viable, so you can decide between boosting Dex, Str, Wis, or Cha for character concept reasons. All of the above will basically just grant skills and saves (and init in Dex's case). And if you're starting above level 4, option 3 becomes a bit more attractive than it otherwise would be. But if not, I'm not fond of #3 because levels 1-3 are where Wizards are the least godlike.