There are still a lot of fields not long enough for jets, are certified but with hard to achieve conditions (LCY), or not allowed RJs for political reasons (YTZ).
Dash8-400's runway length is on the longer side compared to earlier Dashes but since it carries 70 pax not surprising. Both it and ATR42 had various issues on entry to service so I doubt either manufacturer can get much mileage on that score.
Where Q400 scores is speed, at 360kt it's 75kt faster than the Q200/300 (q400.com) because of the PW150A engines. Bombardier claim a negligable difference in range over 60 minutes between Q400 and RJ85/735 and a 25,000 ft ceiling.
ATR42-500 manages 300kt cruise at 17,000ft (from ATR's website) with PW127s.