My answer is NO, I don't think it's cruel.
I have used the prong for Roxey, but no longer need to.
I've tried it with Huney and she'll pull anyway so I don't use it on her. I honestly haven't worked with her that much and it's my fault and I totally take the blame for that.
Angus never even really needed a leash, he just stayed right there with you:)
And Bon...so far he has no pulling issues so I don't have to worry about it:D
But, whenever I take any of them out I put their choke/training collar on. Both Roxey and Huney have "slipped" out of their regular collar and I don't want this to happen.