Weapons of Speed have a chance of giving you a free turn each time they activate. You can effectively time-lock the rest of the dungeon by hitting an enemy repeatedly, so long as the runic effect keeps triggering.
Weapons of Speed are very effective against splitting creatures such as pink jellies. They all have zero defence, so any character of level 3 or above will be guaranteed to hit them. If the runic also has a good chance of triggering, you can hit the creature and let it split as many times as is neccesary without it ever getting a chance to attack. In fact, the same applies when fighting any group of creatures, provided that you can hit them reliably.
Weapons of Speed are also good when you are fighting a creature that is being shielded by a spellcaster. If the runic triggers repeatedly, you will be able to break through the shielding and harm the creature before the spellcaster can shield it again.