As a non-pot user, Spice seems to lack the 'fun' of marijuana- the herbology, aesthetics, and equipment.
Very few people enjoy beer or whisky and decide they will start drinking isopropanol or Everclear to satisfy their interest. They dive deeper, going for IPAs and single malts and tend to be pretty single-minded about their preference.
It's a strange argument, and goes back to the question of how Spice/legalization correlate.
People mostly do research chemicals as a stand-in for their drug of interest. I sincerely doubt legalization would cause "considerably more" than prohibition, and I'd like to see any evidence that it does.
This is the 'gateway drug' canard repackaged.