The Do-jet is apparently the same as a turboprop Do-328 with jets installed. Wing sweep is a feature used to overcome the effects of transonic flight. I understand that the Do-jet doesn't go fast enough to be concerned about transonic effects on the wings.
Andy Neill has got it right - we are basically the same aircraft as the prop version. The limit on airspeed is not the wing (it is good up to .84M or so we were told in class) but rather both a structural integrity problem with the tail as well as a severe Mach tuck problem up around .74/.76. They would have to redesign the tail and that's not cost effective - and with a swept wing we would lose our Vref speed of about 102 to 114. Besides, the company is waiting for the migration season so we can save gas by flying form on flocks of geese - if we can withstand the wake turbulence.......
Actually the wing on the 328 & 228 series Doniers is made of 3 trapezoidal planforms to reduce wingtip vortices.
Dornier did a lot or experimentation with this wing planform in the early eighties even refitting a WWII flying boat with this wing.
The idea is to reduce lift distribution across the wing from the wing root to the tip. Less lift is generated at the tip reduces the size of the wingtip vortices thereby reducing drag. In effect you get the benefits of a larger span.
In addition to the Dornier wings german glider manufacturer Schempp-Hirth makes the Discus High performance sailplane with that wing planform.
Similarly even the Boeing 777 and the new 767-400 wing share this more efficient design philosophy of wing taper rather than winglets for drag reduction.