Kinda a broad statement so sometimes not true, some designer stimulants do offer the "positive" effects of amphetamines like concentration, and some don't.

Usually two different kinds of "bath salts", but there's all different types. Basically you have the kind that mimick amphetamines and the kind that mimic the MDMA/pleasure inducing stimulants. Like Mephedrone or MDPV can offer focus and very similar feelings to amphetamines, as they're amphetamine analogues.

But other bath salts sold like A-PVP can be very like a stimulant while also acting like a disassociative so basically makes you crazy. So the problem comes with lack of regulation in general. And of course lack of education and research as some of these compounds haven't been around long.

Some synthetic cannabinoids may prove to be useful in other ways, same with "bath salts" you could way. Just have to sort out the bad ones.

MDMA was once considered a "bath salt" basically. It was a designer stimulant sold over the counter as a party drug. Now after all the demonizing were finding it has benefits.