Key to aircraft choice is surely not aesthetics, but operational capability.
There’s no question that the ATR is more economical but it’s also slower and at a curfewed airport like BHD that makes a difference. By mixing up their network Flybe will be able to get 5 return trips per day from the aircraft if they want where the ATR will likely only achieve 4.
The DH4 also offers a better option on longer routes like BHXAVN. According to GCMap this is 665 miles with a block of 2hrs 20mins. Compare that to Blue Islands ATR on JERIBZ which at 735 miles has a block of 3hrs.
I think Flybe just have a different route network for which there are advantages operating the DH4