I think it would be better if Atma's got rid of the crit. It doen't really fit with the item. Its meant to be a bruiser or tank item but current Atma's doesn't give enough armor for tanks and not enough AD for bruisers and neither of them really need crit . It would brobably be better if it just gave a base AD of about 20 or 25 in addition to the unique passive or raised the armor bonus to about 70 because 45 is a little under wellming. Its only about half the armor of normal armor items.
As for making it a bruiser item, at least a third of the bruisers in game kill things by constantly auto-attacking them, so flat AD isn't the primary stat on all of them - and even AD Casters like Garen still benefit substantially from critical strike chance. Even tanks can put it to some use - Maokai has the highest level 1 un-buffed auto-attack damage (and still has pretty solid scaling per-level). 20% critical strike chance is 20% more damage with auto-attacks, making the only factor that limits it's utility how often you utilize auto-attacks.
Also - I wouldn't compare Atma's Impaler to pure armor items, you should compare it to other hybridized damage/defense items - because it offers damage and defense, and ignoring half of that will always leave you dissapointed in anything even close to being balanced. And there really are incredibly few armor+offense items - in season 2 the only other ones were Madred's Razors and it's upgrades (Wriggle's Lantern and Madred's Bloodrazors). The only AP variant was Zhonya's Hourglass. And those have considerably less armor than Thornmail/Frozen Heart/Randuin's Omen as well.