Since I am a local diver to the OP, I can not imagine diving without my safety gear that includes a spool pre-rigged to a SMB. 95% of the dives it never gets used, but it is there if I need it. Sometimes we practice deployment. I have used it boat diving in 2 feet of vis to find our way back to the anchor. Two weeks ago we dove Arena Rock and the vis was so bad, we hit the bottom and called the dive. The only way to venture off the anchor would have been a spool or even better a reel. I had both, but choose to end the dive and move to a different spot, where we found up to 15' of vis.
FWIW even though the spools come with a 100' of line, in order to get a double ender clipped onto it you have to pull at least 20' back off the spool. This works great since you can use the remants for making all kinds of tied off connections.