"The one thing about the new Redmi phablets that give me pause is their octa-core MediaTek processors, which aren't as highly regarded as Qualcomm's Snapdragon chips."
Seriously? I think you haven't tested out considerable quantities of budget Android smartphones. I have used latest mid-range (with entry level price tags) Androids such as Lenovo K3 Note with MediaTek 6752 chip and it performed faster and at times on par than most phones powered with Snapdragon 6xx/4xx chips. So this Mi having the latest HelioX10 which is surely faster, more powerful than the 6752 is really a bargain for a $120 phablet.
The appeal of these devices is their power and flexibility, not necessarily some killer feature(although you could argue the ability to play local HD media files is just that). I could care less about another cheap crippled and constrained consumer device(ala Roku, Chromecast). Give me something that plays real HD media and can install apps that allow the device to keep up. There is a rapidly evolving market for TV home media consoles as consumers realize their smart TV's are actually poor investments but at the same time they want to continue to have that home media computing experience they've come to take for granted. It's not mature yet which is why there are way too many poor releases, but the demand is there from the same first adopters that took the plunge with smart phones and tablets.