The organisation that I work for use a medium twin on 24 hour SPIFR SAR/EMS tasks. When the twin is unavailable we use an AS350. Although I do feel safer flying with two engines, my main concern is that the single is not equipped for the tasks we perform. Small cabin area, VFR, no autopilot etc.
Single engine VFR helicopters certainly have their place in a variety of roles but when it comes to emergency services work, particularly at night, they have limitations. Combine that with operations over urban areas at night and I feel that a twin is essential.
Most of our pasengers (patients) have little choice over which type of helicopter they are flown in. I think that they deserve the more capable twin.