I like how you don't notice I mention that those two were examples. I also like how you don't notice I even go as far as to cite their AP, hybrid, and pseudo hybrid builds. Like, hell, find Voluge's comment about AP Kog'Maw, so I know I'm not alone in this regard when it comes to mentioning AP variants of the few Marksmen with AP ratios.
As a counterargument to AD/AP building: Hybrid AD/AP, along with other forms of hybridization (Like mixing Tanks and AP) isn't efficient for a large majority of champs, and my suspicions have been that they have always been (at least technically) inefficient, people just didn't know back then. This is a math thing that you will only ever notice if you calculate gold efficiency for item builds, but the general thing to note is that if you build for 1 specific job (Let's say DPS) the more item efficient items for that job become. The super simplified example is this:
Let's say you're itemizing for Auto Attack DPS. If you buy an Infinity Edge, every Critical Strike item in the game is suddenly a lot more gold efficient (for hopefully obvious reasons), and AD and Attack Speed items also get slightly more gold efficient (Because IE's AD and Crit multiplies Attack Speed, and IE's Crit also continues multiplying AD).
And just so we're totally clear and don't pull another Kog+Corki, that was an example.
Champions who are very clearly designed for hybridization (Fighters for example mix Damage Items AND Tank items into their item sets, Corki Akali and Evelynn have the ratios to incorporate AD and AP, Champs like Ryze and Rumble rock AP+Tankiness) can still build hybrid all they want. This is why Full Damage Darius and Tanky Darius and bruiser Darius all work. This is why I mentioned earlier that Udyr technically has like 5 viable builds. Hybridization isn't useless now, you're just (usually) looking at the wrong champions.
AP Jax and Xin Zhao I suspect is more because people just don't know about them. I mean, and I'm not even kidding by the way, there are a lot of players I've had the...pleasure...of talking to who didn't even know that many Melee Carries like
,
, and of course,
and
even have AP ratios.
The fun, or rather, the avoidance of unfun player interaction comes in the form of giving people something to expect. For example, let's just say Ryze has the ability to hyper carry as hard as he can with AD at a similar strength to the way he hyper carries with AP. Let's say you just backed, all you see is that Ryze has
, and you build some MR to handle his abilities. Bam, Ryze builds Manamune, goes AD, and begins to scale off. You just wasted money on MR and are nowhere near any safer than before. Ryze may be having the time of his life, but now the defending player just feels cheated, mostly because he practically was.