If there was another Dark Lord in making, he would have found out the mentions of Horcruxes. Then he would have tried to find the books and of course may have tried the summoning charm Hermione used. Being that easy, did Dumbledore apply any protective charms so that it can only be summoned by people he chose. If not, wouldn't have that been a highly dangerous step?
A few reasons that spring to mind:
Horcuxes are a very rare branch of magic. You sure you want to destroy that knowledge?
The information in those books could be used by somebody trying to create horcruxes… but also by somebody trying to understand them. I’m sure Dumbledore has used those books to learn about horcruxes, and find out how he might destroy Voldemort’s (which he’s guessing might exist). If he destroys the books, that information is lost. You might regret that later.
There’s no guarantee that a future dark wizard would learn about horcruxes.
There have been many dark magicians through history, but only two (Herpo the Foul and Voldemort) are known to have made a horcrux. They’re such a rare branch of magic that many will never have heard of them.
Even with Voldemort, only a handful of people knew he was using horcruxes – Slughorn, Harry, Ron, Hermione and perhaps Neville. They’re unlikely to talk about it much:
- Slughorn doesn’t want to admit his role
- Harry and the trio know how dangerous they are
- Neville doesn’t actually know that these are horcruxes or involve soul fragments, just that Nagini needs to die
As such, the potential risk of somebody knowing to seek out these books for information is small.
Destroying these books may not be so easy.
We know that books can get defensive (fancy destroying the Monster Book of Monsters?). These books contain very rare and dangerous dark magic – it would be folly to assume there weren’t some defences placed by their authors.
It’s probably within Dumbledore’s capability to destroy them, but he has better things to spend his time on.
I believe the main reason is that Dumbledore is like many of us, and believes that destroying books is intrinsically a bad thing, especially for old books that would be hard to replace. Closing the books away from the library is a temporary decision that can be reversed any time, but destroying the books would be permanent damage. He's not the only one who thinks like that. A very similar example is in Umberto Eco's novel The Name of the Rose, in which a character does not want others to read a certain old book, but also does not destroy it.
A secondary reason may be that the books aren't his personal property, but the property of the Hogwarts library, so he does not have the right to destroy them.
(Update) The third reason is that the Dark Lord's Horcruxes were still around and had to be destroyed. Those books may contain important information on what protections the Horcruxes may have and how they can be destroyed.