Lead lines are extremely useful when it comes to certain aspects of dog training so I?m fairly sure the sort of retractable lead you?re talking about could be used successfully.
My own preference is for a double lead that comes in really useful when teaching a spaniel to quarter the ground, and turn.
I also use it on Labradors as well as spaniels when they visit the rabbit pen for the first few times.
It can be used to pull them up if they give chase should they ignore your initial command and stop whistle.
Another area where the double lead comes in really handy is for retriever training when it can be used to stop a dog running off after being told to ?sit? and on short retrieves to also gently encourage it to come right in to you with the dummy.