Why do some champions benefit very slightly from AP??

just played Urgot in ARAM and forgot to change my masteries and runes, so I had AP. Only benefit of this is a slightly better shield making AP urgot (obviously) non-viable. So from a design standpoint then, why does he benefit from AP at all? if played correctly, that 'bonus' should be 0 anyway. I very much like the idea of champions being designed in a way that allows for different VIABLE builds. And there are many champs who could be AP, AD, tank, or mixed builds. But There are still quite a lot of champions who, though they may have one ability that benefit from AP, its not enough to make an AP build viable. like Urgot for example, its not even enough to warrant an AD/AP hybrid or an AP/Tank. So again, why have him benefit at all from AP? Any thoughts? A response from riot would be awesome - just to satisfy my curiosity ;)