I can't see that these are all that useful with the older style cams, which are pretty easy to tweak the timing on. But, at least until I catch on with how to deal easily with the newer, bolt end, ones, I can see the adjustable sprockets being quite useful there. Should speed up setting the cams.

I suppose if you have your own engine dyno, having these would allow you, as a practical matter, to play around efficiently with cam timing. With hot cams you only have a rather limited range of adjustment. They aren't like SC cams, where you can adjust the timing quite a lot without having valves hit pistons.