You can take multiples in a slot. And 12 armor isn't "easy" to kill... unless the opposing army is throwing all their anti-armor at the Dreadnoughts, leaving everything else in the army alone. Rhinos, Razorbacks, and Whirlwinds have less armor (to say nothing of Land Speeders), Drop Pods and Stalkers have the same armor, and Predators and Vindicators are just +1 armor. So I guess those are all not hard enough to kill to justify getting them?
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Also, as an Ork player, I can tell you they bloody well stand out to me, because I have to bolt a bunch of extra close combat weapons, rather than guns, onto my Dreadnoughts to get those attacks. And Orks are supposed to be the close combat army...