This is one of those items that has both positive and negative effects.
In places where it tends to rain frequently or experience high levels of humidity, the mold can be a problem. In areas with a lot of trees, pine needles and leaf stems, well, more issues to consider. In the drier climes, it's going to be sand & grit.
Obviously, this is not the perfect solution for everyone nor will it be practical for every location or for the whole year.
Here in Southern California, rain is not our biggest problem. The trees can be, but I tend to avoid them because of the bird & sap droppings.
I've run a bra on another car, wind deflectors on others and the most problematic issue has been the sandblasting effect of the smaller particles pitting the flat front edges and the heavier chunks chipping the hood, side mirrors, and front facing edges of the doors.
Since acquiring the Avalanche nearly a year ago, I've played around with using it daily and leaving it off between washings. By the third cycle of this routine, it was obvious it needed just as much protection around home as it did on lengthier road trips.
It's a tough environment out there. Take a good look at your windshield and look at all the pits since the last time you cleaned it.

There is only so much we can do to slow this destruction, but we'll never stop it.
As much as I try to leave a decent following distance on the freeway, some yahoo manages to close the gap and create a debris trail.
I also put a Bra on my wife's new CTS. I ordered a Bra for it after 3 weeks. I kept it on and watched to see what the inside edges looked like and what was happening to the exposed flat surfaces.
I took it off to install the recently arrived license plates and didn't put it back on until I washed it again, 2 weeks later.

There were a lot of dings and chips from the short period of not using it.
The bottom line is, nothing is perfect, but anything will help, one way or another.
The above post was not a rant. Just a reflection on my recent and long term experiences.
Regardless of your decision to use any device or not,
Your Mileage Will Vary.
Drive On!