Hmm true it is good for that purpose. I consider GA my defensive item over banshee's most of the time. Likely more a matter of personal opinion then anything else to be honest.

Abyssal vs Banshee's...actually much tougher to justify than Void vs Abyssal.

Yes, I guess Abyssal is marginally better in stats considering it has 7 more mr and that nice +70 AP for a tiny bit less gold.
But Banshee's build gives +375 health over the +7 magic resist. For 7 magic resist to be worth 375 effective HP, you'd have to be facing a 20-stack Leblanc/Annie or something, in which case you're better off just blocking any initiating nuke with the shield from Banshee's.
Also, magic resist reduction is applied before % magic resist, this actually decreases the effectiveness of void staff + archaic knowledge. So while it says -20 magic resist, it's actually more like -10 effective magic resist or something.


TL;DR
Banshee's is still going to be the best pure-defensive item for Morgana. But if you think you can go about while being squishy, then Abyssal Scepter might float your boat as a balanced offensive/defensive item, sort of like Zhonya's Hourglass but with a lamer effect.

EDIT: Thank you so much for comment and T-up. Constructive criticism is so awesome. :D

Nice build, Personally I'd replace the Banshee's veil for an abyssal sceptre but nevertheless a good build.