9. This class doesn't mark, but its AC is so low that its almost like its marking everyone adjacent to it anyways. Seriously, a starting AC of 14 is pretty likely, and by level 30 that's only going to improve to about a 24 unless you invest in chainmail or put points in dex.
AC 24 is pretty inaccurate for level 30 (though your point itself remains intact.)
10 (base) + 15 (level) + 5 (elderhide) + 6 (enhancement) +1 (dex or int) = AC 37. Now, that is still low enough that enemies of his level are going to hit him pretty much all the time. Chainmail bumps it up to AC 43, which is a pretty enormous jump for 1 feat - though it is still low enough to get hit pretty often. If he throws 4 feats at the problem, so he can get platemail and armor specialization, he's looking at AC 46, and suddenly has a 50/50 shot of getting hit by most enemies - he may have had to put aside some nice other choice in return for those armor feats.
I suspect chainmail will be almost impossible to pass up at higher levels. For a barbarian that completely dumps Dex and Int, it makes a difference of 3 AC at Heroic, 5 AC at Paragon, and 6 AC at Epic. (Since level 11 and 21 will boost your Dex and Int.) 6 AC for 1 feat is an excellent trade - I can't imagine turning it down. Maybe add in Scale and Armor Specialization for 2 more points of AC, along with no armor check or speed penalties.
Now, for a character who starts with some Dex, it isn't as urgent a feat. A starting Dex of 14 means at level 1, Chainmail is only +1 AC over Hide. I'm sure the barbarian will have plenty of better offensive choices to spend his feats on for a while. In the end, no matter what he does, his AC isn't going to be so great - but his high hp and access to countless temps (and Resist All from certain powers) will help with that quite a bit. What it won't help with is the conditions he gets plagued by - saving throw boosts are quite valuable for this guy, I imagine.
But back him up with a Paladin and a Cleric - to keep him safe and healed and give him free saving throws when needed - and he definitely looks likely to completely destroy the enemies in his path.