That fan sure helps keep the air moving a bit and lessening the chances of hot spots and cool areas in the incubator. My first hatch with an egg turner was in a styro-bator without a fan. It was my first ever and I couldn't even track the humidity, so it was already going to have a lower hatch rate (50%).
However, we got a fan after that and a hygrometer (is that what humidity measurers are?) and hatch rates raised up to usually at least 80% fairly consistently.
If you get a styro-bator with a turner and a fan, though, you might as well look at a
Brinsea because the price difference isn't as huge. The
Brinsea's fan is built in (and you don't have to attach it yourself). You can buy a turner to go with the
Brinsea, or if you are at home often enough, then it's really easy to turn the eggs because you just tilt the whole incubator. I get better hatch rates with the
Brinsea. Maybe I just got better at hatching, or maybe the
Brinsea is better. I attribute it to the
Brinsea.
But, yes, a fan is definitely a way to raise your hatch rate considerably no matter which incubator you choose.