The problem with Mid-Defensive and Mid-Offensive armors is that there is literally no reason to use them over fully-offensive or fully-defensive armors. In 90% of cases they're a straight downgrade to to FD/FO leans.
Mid-Defensive armors get hit with this particularly badly. With mid-offensive armors you at least get some benefit from the reduced damage output compared to FOs by taking less damage in turn, but MDs offer literally nothing that FDs don't do better. They still have you deal the same amount of damage with 100-procs as fully-defensive armors, but you take 1.125 times the damage of a fully-defensive armors. The only time mid-defensive armors were ever useful was in the early days of modern leans, when there just were not enough fully-defensive armors around to cover all of the elements and you had to take MDs in their place.
That said, I do think that Neutral-lean armors will still have plenty of niches. Vampire and Werewolf subrace armors are to be neutral-lean when not in beast mode (or they're supposed to: they currently aren't, due to a bug), but they provided so much utility outside of beast mode that the neutral lean doesn't even matter if you're not going all-out offensive in transformed mode. Likewise, the Paladin Armor is currently bugged so that the leans provided by Mantra of Alignment don't work, but that honestly doesn't matter because Paladin class is so versatile and powerful that it'd still be the best light armor in the game even if it were exclusively neutral-lean.
Neutral-lean I can reasonably see being the default going forward for any armor that's primarily designed for utility, rather than straight offense or defense. In fact, I imagine that class armors will likely default to neutral leans in the upcoming revamps.