Remember that the codex states that only one unit may be given the upgrade, which in my opinion is a shame. Ard Boyz used to be an Elites choice, but I really need to let the last codex go.
Its not that Eavy Armor on Boyz is a bad upgrade, just a costly one. With a unit of 30 your spending the same amount of points as 3 Mega Nobz or 3 Red Rokkit Buggies. At the same time you need to be mindful that there are a lot of guns that will punch thru that armor without breaking a sweat, a point which kadeton has beaten me to the punch in delivering.
Check out the BRB pages 290-300, these pages list quick reference sheets for the various races (yes some are outdated due to new codexs). Take a look for yourself at all the weaponry which is directed at the Boys which Eavy Armor would save against.
When you do, you probably won't be too crazy for the Ard Boyz, which is not my point.
While some people are against tailoring lists for play, it happens. When my buds know I'm coming to the table with a KoS list they grab some extra Anti-tank, when they know I'm being a Footslogger they grab up some extra Heavy Bolters. Nothing says you can't do the same, and frankly in my opinion if you know you are going against a Nid List that's essentially just Termagaunts and Tervigons a unit of Ard Boyz would be great. Same with Footslogger Tau, as rare as those seem these days.
The big reason why Ard Boyz are rarely used is because the expense, even on a Trukk squad those points can practically purchase a SAG (well not the Mek whose shooting it). But if you have the points and the desire to use them, I say go ahead. I haven't used mine except in Apoc games for quite a while, but I still think they're some of the best unpainted minis I have in my collection.
Peace
~OD