lol.. it seems as though you are comparing a mage to other classes (warrior to be more specific), which is a bad idea. Each class has its advantage and disadvantages, and even within the classes, how you build your character has the adv and disadv. as well.
Here are 2 types of mages you should look into:
The Support mage: This is a mage where you dont give a crap about the damage you do and is usually ice or lightning. With this type of mage, its best to max out your HP (meaning enough INT for the armor you have, then the rest into HP). This way, you will stay alive to tank and tp your party. Also as you mentioned, you can wear crimson or complete and could "possibly" upgrade it pass +9. But like you said again, if you think it only benefits high class armor wearing people, then follow your instincts but once you become complete and doing crap damage, you would rather stuff all those stat points into HP. (i personally do not prefer maxing out INT, because ive tested and it doesnt really stun any better)
The Damage Dealer:
Yes you do sacrifice major AC and HP (depending on your build) to inflict a serious amount of pain. If you are the type that loves to KS or see 1k + damage on your info bar, then its worth giving up that AC because trust me, the guy receiving the damage is crying like a bytch. You complain about a lvl 80 warrior spamming you for 1k but then again, at lvl 80, most people would probably go with the first mage build (great ac and max MP at 80) so he wouldnt be hitting you for 1k would he so you are not screwed my friend.
So its basically up to you, take your pick. Do insane damage, or stay alive longer. But if we are looking towards the future of a lvl 80 mage, you are looking at 4 possibilities: a massive and possible 5k+ HP paper mage that does great damage, a strong complete mage with maxed out Magic power doing heavy damage while tanking people with its good AC, or a wicked complete mage that max out HP to tank full parties, or a nasty Paper mage that max out HP as well having possibly 7k+ HP.
Its how you build you character. you cant have everything (AC, HP, AP), but you can balance it out. (now add a chitin shield to any of the builds above and its just pure ownage.)