... wat? Any AD that's not your champion's natural AD per level growth is considered bonus AD.
Part of the AD on Maw of Malmortius scales based on how low on health you are. Lifesteal causes you to be healed when you attack, decreasing the AD and the lifesteal on your next attack.
At least I think that's what he was talking about...
I don't buy either Mercurial Scimtar or Maw of Malmortius on AD carries. Both of them have their own problems. Maw has the aforementioned Lifesteal anti-synergy and the shield is pretty small and usually instantly or near instantly broken on an ADC. Mercurial Scimtar is far too expensive and is hampered by the large number of uncleansable CCs flying around these days (Vi, Yasuo, etc). I'd rather invest in either a Guardian Angel or Frozen Mallet depending on the carry. Both have the advantage of helping against both major forms of damage (GA through resistances, FM through health), and both have passives that are extremely beneficial for a carry.
I think the only situation where I'd really build Maw period is if I'm laning against Rumble in top lane and I built a Hexdrinker in lane. I'd finish it off later for a Maw in that case. Mercurial Scimtar is just too expensive for me to ever want to invest in, especially since it would always go in my 5th item slot on ADCs (first 4 are reserved for Zerkers, IE, SS, and either BT or BotRK depending on the carry) and there are better options for bruisers like Spirit Visage+Merc Treads.