This would be the typical assumption, though of course, people may do legal highs as an alternative to illegals.
I would also think the police would have higher priorities unless someone were ordering tonnes of the stuff in what would appear to be an intention to supply, say, mdpv as cocaine (especially as it's cheaper).
Methylone, on the other hand, is far more expensive than MDMA (at least in the UK), and needs larger amounts for similar effects so would not be economically viable for selling on as E. Would have thought the police would be more concerned about mdpv in such a case, as it could more economically passed on as strong speed or even Coke.
And if the person (contrary to your assumption) in question didn't have any illegal drugs in their possession/in their house, I woudln't have thought there would be much the police could do to them, at least at the present time (unless there was obvious evidence they were trying to sell on as an illegal substance) But it's a hassle one could be doing without, of course.
I personally have turned to legals as a way of not breaking the law. I want to get British nationality and even a minor criminal record for something like possession could put paid to that. It's not worth the risk, which is one of the reasons I'm so happy about the legal alternatives.
But I also agree with Fast and Fabulous that it will probably only be a matter of time before things like methylone and particularly mdpv are eventually fully scheduled (probably before the end of the year). After all, it could well be that piperazines such as BZP will be fully classified sometime in the Spring when NZ also probably criminalizes party pills with piperazines (especially BZP) in them. mCPP will also probably go as it can be passed off as MDMA.