first, ment why the rapier over the sapara (didn't notice the mab) and now i question to you oraen, why not rapier/fettering? thought the mab/racc/stats still applied but not the weapon skill?
Sapara is for savage blade, rapier is for melee leaden/wf. If you are fighting against mobs weak against piercing, such as bats, and spamming wf or hot shot, then rapier is def a good choice.
2x mab weapon is not always the best choice because mab atoyac exists. The extra 12 agi on it pushes magical ws higher.
Here is my usual main hand choice combo, feel free to give suggestions if I make bad choices:
1) Savage blade and requeiscat: Sapara +1/blurred +1(feel free to change sapara to nq and offhand to degen if you have Gil issues)
I occasionally offhand Odium if I'm closing multi step with a partner, with tp bonus Marian any tp past 1750 is pointless, and it's very easy to get past that with a partner, if you have a war in pt then the cap is even lower. In that case bigger ws dmg=bigger SC and the benefit of odium often outweight blurred +1.
2) Melee Leaden/wf/hot shot, low mab buff, low acc: rapier +1/mab+oat atoyac. If I need more acc then change offhand to blurred +1, if I get tons of mab buff and engage constantly, or killing omen fodders that dies in 1 ws, then change main hand to blurred+1 too.
3) Melee last stand spam or self radiance: blurred +1 x2. Degen +1 main hand if I use savage blade as part of self SC since it's much easier to connect with degen instead of sapara.
4) Shooting with last stand: kustawi HQ+ nq, if I sub war then change sub to omen shield.
5) Shooting with Leaden/wf: Fettering/Mab atoyac, if I /mage then use omen shield for sub.
6) Aeolian Edge cleaving: Atoyac/Rapier+1 I think? any better combo than this that I'm missing?
7) evisceration: Raetic Kris/odium, maybe blurred +1 is better if I need to sit in dt- set or high acc set?
Sometimes I use degen HQ+nq IF I feel absolutely need to get every point of FC for survivability, 2x degen caps utsusemi ichi casting speed, and it's not far behind blurred x2 if my acc is capped anyways.