Is there any reason NOT to use the heat/cool mode of a thermostat year-round?

You could try turning it off. If the house is unoccupied, OFF is the least expensive option.

This is just me but I think you would do better to cap the temperature at 90, even if you set it Heat-Cool.

Heat-Cool will hold it to the limits you set. If you set it to 90, it won't let the house get hotter than 90. Then you set the Heat setting, and it won't let the house get cooler than whatever that is.

But I bet your refrigerator will be pretty darned unhappy at 95. I get that you're not there every day and you're trying to save some money, but 95 vs. 90 isn't a big savings when compared to having to purchase a new fridge.

Also worth considering: install a programmable thermostat, and run the AC for a few hours at night to pull the house's temperature way down. My understanding is that deserts get pretty darned cool at night, which means the AC will have a much easier time of it, and it may be able to pull the house's overall temperature down enough that it doesn't get as hot as all that during the day.

Good luck with it.

uniform humidity keeps wood in the structure supple,well fitting and prevented from buckling as the other guy stated

You could use the heat/cool and set it to 95, but you need to understand where the thermostat would attempt to energize the heat.

No. No advantage of either setting.

BUT, letting the house get to 95 or even 90 degrees seems pointless. You may as well let it get to ambient temperature. The real cost comes in when you cool the house for comfort. If it is 90 or 95 degrees to start with will make little difference.

cool only would be better.

Heat/cool would try to keep your house at 90 when it was cooler outside -- using much heat...

I can't think of a reason to keep it any cooler. Moisture is your enemy but that's not a problem. Mold needs cool and moisture that a dry 95 degree space will not support. No warping either.

Keeping it so hot for a long period of time can damage the interior of the house. It can get moldy, floors can buckle, damage to paint, etc. Most recommend 85 as the max temp.