Thing is. When do you want to build it? The biggest problem for mages right now is early game. They are completely smashed in lane by assassins and fighters. Even rushing Zhonya is just noob trap, that little armor it provides is halved by lethality they get from serrated dirk/duskblade/ghostblade+sudden impact and as I said before, without hitpoints armor is meaningless. On top of that you will be doing so low damage that other than you survive maybe 0.5 second longer you just make youself free kill target.
What mages wants mainly besides AP is CDR, mana and hitpoints. If you look at armor items, only 3 items matches the criterias:
,
,
- ohwrecker has best base stats but its useless active makes this item a no-go. Knight's Vow's stats are worse than Zhonya. That little hitpoints are not worth it. Rigteous glory has even less armor than Zhonya and its active is again unusable by most mages.
Really, the only real option that actually does make a sense is
but that item is missing cdr and also mana. Rushing this item is really not a choice because not only you don't get higher damage from your abilities, you will also get no CDR (which would essentially increased your damage and defense indirectly) and no mana so you end up oom after 2 trades.
Mages suffered many nerfs and almost every mage must rush
or
or
, sometimes even combination. Rushing anything else, you are going to have big mana issues.
The worst thing is that even when you build that deadmans, randuins or thornmail you still get steamrolled by full ad anyway (been there done that). So I am not sure if it has any sense.. Again the biggest problem is early game where building armor will not help you because lethality and low hps. So this is just lose-lose situation.
If I see full AD teamcomp, I rather take malphite into middle rather than mage.