Well yeah, I was more wondering why so many clearly manaless champs are mana champs in the first place. I guess it's just the default resource management system in the game so meh.
But Spear applying Lifesteal would be no more special than GP's Parrrley applying Lifesteal; which it already does. As such, the concept, I feel, is not inherently overpowered.
Kat's AD scaling is actually better than her AP scaling (outside Shunpo, but Death Lotus and Bouncing Blades have strong AD scaling); the only reason you build her AP instead of AD is the fact that you need Spellvamp, MPen and in general, stats you only get from AP items. So in fact, the same exact problem we're talking about in the first place, only in her case the solution is "make her do magic damage" apparently.
But yeah, I feel she's a holdover from the time when abilities dealt magic damage period, and should be changed to the more modern design philosophy. Autoattacks I have no real problem with though I would prefer magical autoattacks of course.
Well, I feel there's no reason for game design and sense to be in opposition in this case. I feel it would be entirely possible to change all current AD-scaling abilities to utilize Lifesteal and Critical Strike and, with some adjustments, maintain perfectly good balance while suddenly enabling the current itemization to serve all the champion categories.
Some changes would have to be made, sure. But that's, I feel, not nearly as heavy-handed as the changes that would need to be made to make AD caster itemization work with the present system. I feel even the old Gunblade was a bandaid fix at best; even then you wasted the AP stats and mostly the Lifesteal stats most of the time and that made me feel it really silly whenever I built it on champs that don't have solid AP scaling.