Casting Void Blast extends the duration of your Rift which in return keeps your Mind Blast to be Void Blast longer.
Optimally you would stack up 3 Mind Melt stacks before using Void Torrent to start with a juicy Void Blast crit. Then your priority is to keep Void Blast on cooldown but before casting one, you need 2-3 stacks of Mind Melt from Mind Spikes. You can only do that realistically if you have Distorted Reality talented because it is quite impossible with the base Devouring Plague duration.
For me, that feels OK in the Raid but I don’t like that slower playstyle in Mythic+ at all. Although Mind Spike is definitely more favorable with Voidweaver on paper, I enjoy Mind Flay way more with Mind’s Eye as it is a much faster paced rotation and gameplay.
Maybe I won’t reach a 100 parse but even towards the higher end I’ll always choose the build that I enjoy the most. At least if it is not considerably behind.