So we all know that Voldemort made a lot of dumb mistakes, particularly where Harry Potter was concerned. His obsession with the boy, and with doing things right once he had the chance to kill him, ended up costing him time and time again. Talk, talk, mock duel, monologue, threaten, show off his power… Others like Cedric didn’t get any of this special treatment, and so Cedric died.
One area where you have to give the Dark Lord some credit, however, is his Horcrux magic. That was smart. He knew how important it was to keep his soul-pieces safe, and put in the effort, hiding them away in well-defended places that only he knew about and that would be almost impossible for others to track down. More than that, he even used them as offensive tools, as well as to defend his immortality. The locket could influence its wearer. The diary would reopen the Chamber of Secrets. Nagini could go and do his bidding.
…Except no. Once again, Tom Riddle was too concerned about the grandiose nature of his horcruxes, the significance of the objects and the power of their defences. Once again, he overlooks the best and most powerful way to use horcrux magic because he is too vain for his own good.
So if Voldemort was truly diabolically smart, and wanted to make the absolute bast use of his soul fragments, this is what he should have done:
He needed to horcrux human beings who had been subjected to the Dementor’s Kiss.
He was allied with dementors during both the first and second wizarding wars, so he had the materials he needed. He just never made the connection.
We know that animals (and indeed, people) can be made into horcruxes, but that the presence of another soul in the body prevents the horcrux from taking control in all but the most minor ways. So if the person’s own soul has first been sucked from their body by a dementor, then the horcrux would be the only soul in that body, and Voldemort would have complete control.
Then, since magical power is tied to the soul rather than the body, he would be one person in two bodies. There would be two equally powerful Dark Lords running around. Repeat the process a few times until he has his desired seven-part soul, and you have seven Voldemorts, all with the power of the original, all working together for world domination.
After his first defeat in the books, it took Voldemort 10 years to recover even a tiny part of his strength. He was “less than the meanest ghost”, slowly clawing his way up the food chain until he got strong enough to be able to possess Quirrel. But if he had been cleverer, he wouldn’t have needed to.
So, when one Voldemort gets blown up by that stupid love-magic and a one-year-old child, he is still able to work through his other bodies. Another one can be called in to rescue the one that got exploded and also to kill the infant for real this time. Another is off killing Neville Longbottom, the other possible subject of the prophecy. He doesn’t have to go for one or the other first when he can be in seven places at once, doing seven different things.
And then in the second war, imagine if one Voldemort went off hunting Gregorovitch and the elder wand while another stayed home to command the death eaters. Another can oversee the ministry’s downfall, while a fourth can dedicate all of his time to hunting Harry Potter. One Voldemort had to fear Albus Dumbledore, and they duelled to a standstill in OoTP, but seven against one? Goodbye Headmaster.
This also means that the horcruxes are much easier to defend. There is no need to put up a bunch of protective enchantments around each and hide them away, never to be found. The horcrux-bodies can defend themselves using Avada Kedavra, and all the other powerful magic Voldemort can perform.
Voldemort allied with the dementors during both of his wars, so he had the materials he needed. It just never occurred to him to make use of them in this way. I think we can all agree what a good thing this is. If he had made the connection, it would be All Hail the Dark Lord.