It kind of depends on the champion, and the time that the ratios were added. Older players like to talk about how in the beginning of League, attack damage increased the damage of attacks and ability power increased the power of abilities, even on champions that were intended to mostly auto attack. (In fact, attack damage still increases the damage of attacks even on champions that mostly use abilities, so this is still mostly how it works.) The oddity, then, is when an ability scales with attack damage.
Except that that's not odd anymore. At some point, Riot decided that having champions whose abilities did more damage based on how much attack damage you had was a good idea. Now we have many of these.
And on some champions, hybrid builds were definitely intended, or at least seem to have Riot approval. Champions like Kayle, Jax, Akali, and Kennen were all probably intended to be hybrid champions.
On some champions, the ratios inspired alternate builds. Master Yi's Q used to have an AP ratio, and AP Master Yi was a thing. AP Tryndamere used to be a thing. AP Varus and AP Tristana are sometimes things. These aren't/weren't even hybrid builds (although hybrid Varus is also a thing). Curiously, AD builds on typically AP champions seem to be rarer than AP builds on typically AD champions.
There was also a patch note a year or two or three ago (I don't remember when. I could probably find it by looking at the LoL wiki's record of Nocturne changes.) that gave Nocturne's passive an AP ratio, not to support a hybrid of AP Nocturne build - Nocturne is definitely an AD champion - but to make incidental AP gained from sources like the Baron Nashor buff or the Natural Talent mastery (which he might take for the AD anyway) not feel wasted.
supp don't agree