I think we're missing the true value of rune weapons in this discussion. They are a living record that stretches back for millennia. Rune weapons are conscious, aware of their surroundings, very old, tend to be in the hands of some very interesting people over the course of many thousands of years, and they tend to be loyal and obedient to their wielders. A 10,000 year-old rune weapon has experienced the equivalent of 200 full lifetimes, and unless it got shoved into storage somewhere, it's been either wielded by or close to some very powerful people.
Their vast experience could be useful in all kinds of ways:
Old Ones Spoiler below:
+Rune weapons tend to be privy to their wielders' deepest secrets. They could guide a worthy wielder to long-forgotten treasures hidden away long ago or take advantage of still-sensitive secrets.
+Rune weapons have fought all kinds of horrible enemies. They could give excellent advice on how to defeat all kinds of enemies like demons and devils.
+Rune weapons have been through places that are now ruins. Their memories could help a wielder find his/her way through ancient ruins, avoid ancient traps, et cetera.
+Rune weapons have seen wielders wield all manner of magic. They could help a wielder rediscover secrets of long-lost magics.
+Long-active rune weapons have interacted with many cultures and could give a character insight into situations that no ordinary character would otherwise have.
I'm sure there are other ways to use the incredible knowledge of multiple millennia. When you think about it this way, an experienced lesser rune weapon could be far, far more valuable than Castledrake, who has spent most of his existence disassembled and presumably unconscious.
Imagine if some rune dagger got crafted in ancient Sumeria for a king, then presented to the ancient Egyptian royal dynasties for a few thousand years, then passed on to the Greeks, taken by the Romans, sold to the Phoenicians, captured by Vikings, captured again and taken on the crusades, given as a ransom to the Saracens, captured by the mongols, sold to Marco Polo, taken to the New World by Cortez, captured by Blackbeard, taken by Horatio Nelson, given to the Duke of Wellington, passed on to his descendents until World War 1, where it was captured by Rommel, who passed it on to his son upon his death, and so on. The value of such a dagger would be incalculable! Who cares if it can spit fire a few times a day when it can lead you to King Tut's tomb, translate heiroglyphics before the Rosetta Stone was discovered, tell you what happened to Caesar's missing treasure (and guide you to its hiding place), tell you how to make Greek fire, Damascus steel, reveal the secrets of Viking navigation, tell you what the Templars actually found in Jerusalem, et cetera?
This is why I see rune weapons as being so incredibly valuable. Where a petty adventurer might just see a super-invincible sword that he can talk to when he gets bored, an inquiring, curious, ambitious wielder sees a base of knowledge he can use to catapult himself to outrageous fame, vast fortunes, defeated enemies, far-reaching influence, and/or any of his/her goals.