I started doing this after running Dzemael Darkhold and using both to help get off some extra uninterrupted spells during the second boss (boss had fire based aoe and adds hurled rocks). Never stopped using it since for 2 reasons...

1) If you need both, the benefits are obvious (not often as you said, but the combination helps slightly with titan and ADS in coil, amongst some quest mobs)
2) If you don't need both, you waste nothing by popping the other (macro executes in 1 second and gives me an extra space on my hotbar, which for ps4 users like myself, is nice)

But I've learned to be a bit smarter....I put manawall first because the cd is fairly short relative to manaward. If i need manaward, I let the macro execute and have manawall as a bonus (unnecessary maybe but not inherently bad) and if I only need manawall, i just interrupt the macro before manaward is cast. Complicated, maybe, but I've done it long enough that it's second nature