We have an AD/MR why no AD/armor? It would be for full squishy bruisers, for full tank bruisers, for poke champs, for assassins. It just makes sense. If I'm getting pooped on in lane by an ad opponent, but I scale with AD very well I can pick up this armor/AD item. But their isn't one. Are you scared that it would make those champs to op? I don't think it will, and if it does then all you have to do is balance. Please Riot.
It would either be undertuned or become necessary as the first item in bot lane to be able to trade with the enemy adc.
We have Frozen mallet for ad, a tonne of health, and a slow on hit. That works pretty darned well for what you're looking for if it's later in the game, and I don't think an adc getting early game armor is very logical. Pick up Ninja Tabi if it gets really bad I guess (or just grab a cloth armor and sell it later).
Late game, Frozen mallet works for general defense when you're still looking to grab more damage. I've actually build Randuin's Omen on Jinx my last 2 games to deal with fed enemies (first against a 23 kill Fizz that had Trinity force, BotRK, IE, etc..., [I was the most fed on my team and Randuin's let me survivie him in teamfights, then clean up for the win] second the enemy team had a crit-plank and their adc Tristana as their most fed champions [I had a pretty significant lead that time, but even with Randuin's I still needed to use heal when I purposely 1v1'd GP which was unexpected, then again he had about 15 caster minions with him]). If the enemy team isn't stacking armor, there really isn't anything wrong with picking up a defensive item 4th (IE > PD > Berserker Greaves > Defensive > BT and LW), even if [or maybe especially if] you're ahead.
I'm assuming this is because ADC can abuse this item. If you're ahead, you can buy the AD/armor item and not let their ADC do any damage to you, etc.
Because Riven exists.
Build out of Brutalizer and a Chain Vest.
Gives you a short range aura that steals 10 armor from each enemy champion near you. That would make it effective at diving into the middle of the enemy team, but not so hot against in more standoffish situations. So long as the aura is less than the average ADC's auto-attack range, it wouldn't be attractive.
Or.
Y'know.
Make it melee only.
Because Bruisers will abuse it insanely hard, and with the new items this is the last thing they need.
There needs to be a deeper reason to have an AD/armor item beyond "I'm laning against an AD bruiser", since that is not only the staple in top lane (so the item could become mandatory in practically every game if it were any good, particularly since it would also offer resilience against marksmen), but also runs the risk of creating an arms race, where whoever gets the item that gives more damage and counters the enemy's damage wins lane. 
did a good job of shortening the list of champions it was viable on by not offering raw AD from its health and relying on autoattacks, and I think if Rito were to create an AD + armor item in the future, they'd also have to make it good only on a specific subset of champions to avoid ending up with another Atma's Impaler (which was either completely OP on every bruiser ever or totally useless depending on its numbers).
nuff said
We need more MR items before anymore Armor items.
