Far be it for me to have an opinion
Since I don't have a listening device installed in my home, but surely one of the useful things about online shopping is the ability to see what you're supposed to be getting? To browse the reviews, particulary the one-star ones to note potential issues, and to perform a direct comparison between similar items? Giving instructions to a speaker might only work if you aren't that bothered what actually arrives. One might say "Oi, thingy, order me two packets of pancake mix" but one would never say "Hey, thingy, order me a new fridge please".
Not to mention, the inherent creepiness of a device that's always listening and may or may not be passing recordings back to the mothership.