There is a lot of, "you should be doing this" and not why you should be doing this, so thats what the OP argues. In this case Manawall/ward have no fight that I can think of you need both at the same time. Howver in most fights i'm using both of these as much as possible. The reason is like only 1 person said, manawall will prevent knockbacks(not the knockback from titan ex goal stomp though that's the only I can think of) It will prevent a knockback from landslide which is about 2 extra cast a use. if you use it every-time it's up to not move during landslade
1)For the OP. That half second makes a huge difference quite often, there are a good number of fights where if you don't have the proper spellspeed, and/or hesitate/loose .56 seconds you are going to interrupt your next cast, or have to cast a dramatically weaker spell, and adjust your rotation to keep dps up. Right after titan lands at hear phase for instance eI can a thunder 1 -> am -> fire 3 off just before I have to move for that first set of plums, however if I miss that just half a second at anytime I wont get both spells off. that's a dramatic dps loss for blm. AM is a gap closer, I have mine set to a certain party number and I put the person I will be porting to for that fight as that number, ussualy the drg bards jumping backwarsd when you use it is not fun.(though I have levi one that's for my tank so i'm always on the proper side). You can use it on titan/ifrit ex to stand back for the first 15seconds and nuke while the dps waste time running in then just AM over to them. t1 you can use it to help dodge poisen spits, t2, -t4 don't really have a lot of movement I have only used AM in T4 way back in the day. T6 yep, levi, yep. so basically 75-80% of the current content it will provide you with extra dps.
2) VIVI are you a troll? why are you starting with fire 1?