I'm not sure if it mattered how much AD/AP, but I know that as a Mage/Marksman, the only AD I had was my base stats, runes, and masteries. I was originally trying to do an ADC build with Kennen (top), before getting shafted in lane and getting ganked multiple times. Full tank (it's been like 2 months, but I didn't even attempt getting MPen or APen items; just straight tank) with like Thornmail, Sunfire, Randuin, Banshee, Visage, and Tabbi. I had reasoned that since I was a yordle champion, and the enemy jungler singled me out several times despite being pushed to my tower (over, the much more dangerous mid who was getting the same fate), that going full tank to deal with the full tank Gnar would be more beneficial to the team overall than an Anti-ADC build, which is what I normally run for my Kennen builds. So for a Mage/Marksman, I had very little actual kills (discounting KS's) and a gigantic number of assists. People would probably target me like they go after every other yordle champion, so massive tank and AOE charging CC would probably allow my team to win the game. We did, by acing them late game in our base. Like all Bronze to Platinums, they focused the tank, who proceeded to five man ult them and for the rest of my team to jump into the fray.
My concern is playing a champion to a role they're not officially supposed to be in. For example, Blitzcrank is a Fighter/Tank. So using him as a Support (as 99.99% of Blitzcrank games go) may push your Fighter and Tank mastery up, while not touching your Support mastery. Or using an actual support as a fighter or marksman (during the Nemesis mode, my team ran into an ADC Soraka who proceeded to crush us mercilessly).