Actually, BKK-AKL/FRA/CDG/LHR/ZRH/CPG... will get about 11,000 miles in 1 go and thus ROP S / *S.
Regarding BKK-JFK on THAI - of course, they'd need to accrue their miles to THAI to become ROP Silver, which might not suit all passengers...
However, the benefits are ... well ... not much. If you fly THAI as ROP S you get an extra 10kgs baggage allowance, which is nice. And a nice 3000 mile bonus. But that's it.
On attaining Silver status you will receive your personalised membership card, a Member’s Handbook, and 3,000 threshold bonus miles.
Exclusively on THAI
Priority baggage handling and an extra 10 kilograms baggage allowance (except where luggage allowance is determined by piece) on all THAI flights.
Star Alliance airlines
Priority reservation waitlisting and airport standby* on all fully booked flights.