Flexible dates are a nice thing to have but I wonder how much they would actually be used? I cannot think of a time where I was booking a flight for business and not only were the departure and return dates non-optional but I was looking for pretty specific times as well (i.e. within a few hours time range).
Even when traveling for pleasure I generally want specific dates so as to maximize my time at my destination.
The problem is that each person has different needs and it's difficult to make a site that is usable for most of them - that's why so many 'niche' sites exist.
For example I used to fly a lot and most of the time I did not care about exact dates at all. I guess that scenarios like 'I live in Spain now and I want to visit my parents in the Czech Republic some time in June' or 'I want to spend some weekend in Paris' are pretty common.
Sometimes I don't even care about destination that much - I want to travel somewhere in south of Europe to take a couple of days off - all I care about is the price. Date-of-flight-centric sites are really difficult to use for me. Only recently I discovered http://skypicker.com (I'm not affiliated wit them) that does what I want - I want to fly to Spain some time in August - here are cheapest flights.
Even when traveling for pleasure I generally want specific dates so as to maximize my time at my destination.