As has been said several times before:
If you are in a off road vehicle, doing off road things, wear the helmet. Wear the belts. Wear the Neck Restraint. Wear the suit.
Part of "testing" is getting to know how the car feels right? If you aren't wearing what you are required to wear on race day, then how the vehicle feels can be completely different.
There is also a higher risk potential testing, pre-running, or even recreating when you aren't out at a sanctioned event. You have zero people standing by to help. You also have zero control over the other people that are out there doing the same things you are.
If you are in a crash in a caged vehicle, the cage will kill you dead faster than a non caged vehicle if you are not properly prepared. A few years back, Pat learned this the hard way testing in Barstow. This is what a rollover in a caged vehicle without a helmet can look like. It can also look much worse.
I have also seen some teams practice pit stops without suits because it was to hot, or didn't want to get the suits dirty or damaged. Dress the same exact way you will be on race day each and every time. You want to makes sure you can lift, kneel, bend, and move properly, regardless if your perceived threat is reduced because you aren't at a race.