Your ignorance of the Twotter's unique capabilities are disgusting.
I guess the only thing that would cut it for you would be a shiny B777 doing .84 from DXB to JNB.
Every airplane has unique characteristics that make it appropriate for certain missions. The Twin Otter is legendary for its short field performance, as well as working out of rough airfields. Its carrying capacity is solid, and it has a good safety record. What Viking and MM has done in updating the design for this century is outstanding.
Twotters are working across Africa, doing everything from helping out with oil exploration to humanitarian aid missions.
If you want to rip on the DHC-6 for only doing 150kts, well I can assure you that some of those guys flying the Twotter for Zimex and other oil companies are making similar salaries to folks flying those sparkling Airbus and Boeing widebodies in the Middle East.
GW