There are operations that require twins due safety, IFR, hostile terrain etc. There are operations that require singles due no infrastructure of any form thus no tech support/hot and high where twins especially light and medium twins just do not have the payload of a single(ie 350B3) on single engine performance and thus becomes a expensive single/funnily enough due to extreme hostile terrain requiring high performance and range.
Helicopters are extremely versatile and so their application! The argument supporting twins for say SAR/IFR ops becomes the same argument supporting a AS350B3 in the middle of nowhere doing geosurvey at 6000ft. 38+ degree C with a 1.3 ton loop under her 8 hours a day 6 weeks in a row?