I think the reason people like them is that with most midrange tyres such as Falkens, Kumhos etc, the main thing you lose against the premium brands is wet grip, and with the rainsports you don't.
The issue I have with them is the the sidewalls feel quite soft, which you do very much notice in the dry compared to the market leaders, but then again you have to drive harder than 99% of the general public ever will for that to become noticeable.
A lot of people who buy them will be coming from tyres like the Dunlop Sportmaxx or Pirelli P Zero, which although premium, aren't anywhere near as good as something like the CSC6 or MPSS.
I agree that anyone saying they're as good as any of the latest premium offerings is talking rubbish, but you can definitely see the appeal.