Yes, generally, its inefficient to combine tools to get to higher levels, but I still do it on cheap lvl-1 enchantments or diamond tools gotten through trading. And regarding the Sharpness vs Smite, I like both, Sharpness as a sword to just carry all the time, except when hunting wither skeletons and fighting the wither. That´s where smite comes in.
I read the link that you provided and i was shocked by the fact that it says "Sharpness is useless on diamond", so I tested it myself on zombies and creepers. On zombies it made absolutely no difference because on their tiny extra armor they naturally have. On creepers, though, it did make a difference, lowering the hits required to kill to 2-shots. Smite on axe, on the other hand, one-shot zombies and was no different than sharpness on creepers. Well, thank you for showing me a different and possibly better way of gearing myself.