Season 2 meta: AP is all burst, we don't need to worry too much about them as long as we can survive it. Let's get 1 merc treads, then stack HP for the AP and armor for all the AD.
In comes the on-hit items: malady shreds away that merc treads MR, while madred's eats that whole pool of health thanks to the synergy of attack speed. bloodrazor and wit's end provide decent defensive stats, while a frozen mallet can be added for HP and utility. If you're feeling particularly precocious, nashor's tooth should just about max your attack speed while providing a nice AP scaling to the ability you PICKED on-hit for to BEGIN with, along with CDR for when you need to keep your buffs or CC going. You have some hefty damage, proccing a ton of magic on each hit, while your base AD plus the bloodrazor is far from negligible, and your fully stacked attack speed can attest to the dps. The only thing that can resist you is a fully stacked defensive tank, say over 200 each of armor and MR instead of health, but that takes a lot to build and isn't meant to be focused to begin with.
Then you have the arguments in favor of BRK
"It's physical so armor penetration works"
Oh, so now you need to build a last whisper for it. Great if you're an AD carry.
"The stats are more streamlined"
AD and lifesteal do together, but has nothing to do with the proc. To make any use of the lifesteal, you're going to need a TON more AD in that build. Streamlined? More like AD carries only club.
And did I mention it's ALL AD? See that thornmail? there goes ALL the healing it would have added and then some!! stacking armor? Even if you DID spring for the penetration you aren't going to get far, armor is cheaper than MR and those champs have a good pool built-in.
So let's see, you NEED to buy a LW and some AD, let's say cleaver with it. Wow, 30 minutes of gold, glad I have this proc to use, and look at all the attack speed these mandatory items provided.... aaaand I have none. Halfway through the game and I have zero attack speed for this on-hit proc. Let's not forget the progression of focus: The burst comes FIRST. Suddenly current health is a lot lower, glad my damage scaled down to HALF, along with my best chance to recover my own health.
Madred's was a bit expensive, but it was SO much better than this piece of junk. Even the AD carries it was intended for can barely use the thing, while on-hit champs get ZERO value from it.
A simpler, streamlined item IS a good idea. Let's get some synergy while making sure it's not 100% useless. Replace the cutlass with a stinger, the proc SCALES with attack speed. Switch it back to magic damage, the meta is STILL health stacking and low MR, don't FORCE an armor penetration item, and leave it as an alternative to bypass thornmails. Finally, just make it max health. I LOVE the idea of a healing/sustain item for on-hit. That is the ONE thing BRK did well. Everything else about it just locks people into mandatory builds, and denies the champs who would WANT the item from buying it. You wouldn't complain about liandry's mushrooms if on-hit teemo actually had his build options instead.
Let's get real, Madred's was the core of on-hit, while BRK is for ADC only, and manages to fail even that.