I think it depends how many things you are procedurally generating. If you are generating species of plants, types of boss monsters (like demons etc in Dwarf Fortress), civilizations at war and peace (like in Dwarf Fortress), then it's a waste if players CANNOT re-enter the world and experience those things in another way. Each playthrough feels like a new chapter in the saga of that civilization or whatever.
On the other hand, if the only procedural generation is, e.g., the correspondence between obfuscated potion names ("swirly potion", "pink potion" etc) and their actual effects (invisible, poison, etc), then there's no reason to let them re-play it. Nothing is gained but something important is lost.
