TBH you can either go semi-tank or glass canon with Kass, it just depends on you're play style.
When you play semi-tank you do a bit of sustained damage and when people run away you seal the deal through a combination of blink, silence, and slow w/ light mana drain.
When you play Kassadin, you jump into team fights, take out one squishy and run. Often time you can divide the enemy team too, getting some to follow you (if you're not in a high tier game that is).
Both styles are viable, although the glass canon makes farming (champions) easier, and the tank-ish build is a bit easier to play. I personally run a 1/2 and 1/2 build with an early RoA followed by a DFG/Void Staff/Abyssal Scepter. Almost universally, Kass builds take Catalyst early anyways, so why not build it into something useful?
The second RoA seems a bit like overkill to me though... if you buy it at like minute 30, it will never build up to its full potential (and therefore it is an inefficient item). For the same cost, you could buy Rylai's, which is like a maxed out RoA w/o mana that adds a slow to void sphere and riftwalk (which is actually pretty good for chasing, ganking, etc).