You left out the part where they stink like stinky cheese, require extensive bookkeeping, rewrite the Knowledge check rules, and make players cry.
That's true, but I love the IDEA of legacy weapons. I love the idea of characters and weapons being intrinsically related. I want players to be attached to their gear, not just gaff it over the second they see another blade with a better enhancement bonus.
I also like Legacy weapons because they can allow for magic items to be a bit less common - and cinematically, the descriptions almost write themselves!
"And lo, here trod Cairn the Demonslayer, whos axe Hornsmiter was quenched in the blood of the Demon Lord Anrethsh and from it burned with the very flames of perdition."
To me, as both a player and a DM, that kind of organic enhancement would be awesome. I've always hated that any significant attachment to the gear I started out with is useless, because in 3 levels I'm way behind the curve if I haven't traded up for the next more powerful weapon in the arsenal.
I'm all for Leagcy-esque weapons, and hope that they are core. I -do- hope they revamp the requirements for Legacy weapons, though. As noted, they're a mechanical and bookkeeping headache. I'd rather see them simplified a bit, and at the very least not be mechanically inferior to the randomly generated treasure one might find in a horde. Give me a reason to keep the sword of my grandfather, and I will. I could see some kind of system similar to Eberron's Artificer recycling - bringing your current weapon and some 'better' magical item to a wizard to have the power in the new weapon given over to your personal sword, or even a feat tree or something for wizards or combat classes that would, given enough expenditure on materials, allow the player to migrate the bonuses themselves.
Heck, I'm likely to house rule such a beast into existence if it doesn't show up pretty quickly in the core rules.