Hi Graham,
It has been my impression that these 'pet rocks' (never heard them called that before
) are designed more for the inshore reefies, like groupers and cod and particularly pink snapper/sea bream.
This is due to the fishery in Japan trying to spread its pressure from just targeting the Kings and AJs to other more accessible, less-fished species. These reefies are generally less aggressive, and the 'pet rocks' work on these species very effectively. Their lack of size or ability to put up as much of a fight as their seriola or tuna counterparts is why 'pet rocks' come with much weaker and smaller hooks, compared to the large assists of metal jigs.
I am still convinced that if targeting doggies, AJs, YTK, mackerel, tuna, etc (hard-running sportfish) then metal jigs are first preferrence. For reefies, you have either option. Having said that, metal jigs are what I have spent considerable time and money playing about with, and that is still my preferred choice, with results at the moment good enough for me. It has been on par with bait and 'pet rocks' for reefies like coral trout, red bass, chinaman, cod, reds, etc.
If you really want to get into it, 'Pet rocks' require a whole different approach and perhaps a completely different tackle set-up... possibly the reason why I am reluctant to switch. Its a case of slow and steady vs. the swash-buckling allure of metal jigging.