For Amumu and Soraka, I definitely agree with you, which is why I was making a comparison based on the item's reduction alone. Some champs will just do better with Abyssal because it suits them (my good friend Fiddlesticks comes to mind). However, keep in mind that the aura is useful if you have others who deal magic damage on your team, so in Wikey's match, it would've been useful. However, Katarina, Amumu, and Soraka still make better use of it than Brand, because the first two will almost always be close enough for the aura to matter.
The aura is also easy to walk out of. It's 1000 range, so someome at the edge of Brand's W won't get the debuff. In addition, his ultimate bounces, meaning it can easily bounce out of range of the aura, unless you run after it like a madman. In addition, you have to examine whether or not MR is useful to you at that moment. In Wikey's match, Lux was the only real problem, so the MR wouldn't have been so useful. In addition, the enemy team had 4 melee champs, whose MR goes up naturally. Then consider the probable MR runes, and Shyvana's ultimate's passive, and the 40% MagPen becomes way more useful.
Now, on a burst mage, raw damage is extremely important. The reason for this is that, during a team fight, the burst mage uses all their spells, then runs out. Brand wants to use his spells together to make the most out of their extra effects, then gtfo. Burst mages typically build one or two survivability items (Rylai's and/or RoA) because they still provide good amounts of AP, and help survive the onslaught of the enemy team. I would caution against building a RoA late game, as you aren't getting all of the bang for your buck. Zhonya's, while an excellent item, is in fact situational, being a greater priority if you find yourself dying a lot (or if you have an AoE ult that isn't hard channelled). WotA is nice, and I believe Brand's passive works with spell vamp too, so if Kat or Soraka didn't pick it up, he definitely should have. DFG is also situational; if they happen to have high health champs, and you have single target burst, then it's useful (think Veigar). Or if you happen to be Swain, lol.
You're definitely right though, he should've had a Deathcap first, so I don't know what he was thinking. Still though, a Void Staff is generally one of the stronger purchases the average AP caster can purchase, due to its cost efficiency scaling with the MR present on the enemy team. Individual casters have to be considered case by case though, because kits and circumstances change a lot.
