Just speculation, but I would guess that it’s because you want your car to look good, and to be as good as it can.
On the broader issue, I get the top down as often as I can, even if it means two fleeces, a hat, a scarf and gloves in the depths of a Cornish summer. My MX-5 is my daily drive, and there are times when I need to arrive at my destination looking (as far as possible) smart and presentable. So sometimes I drive with the roof up on a dry, sunny day. Coming back is a different story. For similar reasons I have the hard top on during the winter months, but then it’s only the work of a few minutes to take it off and stow it.
I think that MX-5s look good with hard tops on, with soft tops up and soft tops down, but I also believe that the “natural state” for a Five is with its roof down. It makes sense to me to raise the roof when it’s parked, not least because of the morons who will damage other peoples’ property merely for the sake of doing so, and also because we have a lot of ill-mannered gulls in Cornwall.
As far as distressed hair is concerned, I have so little left that it’s not a personal issue, but looking at some of the earlier comments it is now quite clear that the MX-5 is not a hairdresser’s car. If it were, then the drivers could sort out their own hair at the end of windswept journeys.