For myself, and others I play with, the answer is it depends..
If we are in a Wing, undermining or fighting in an expansion system - we fly in open. We are the threat, hoping to run into enemy commanders that we can kill to take away any merits they have gained before turning in. If we are sniping a system, we will fly in a private group - so as to not alert our enemies of our presence.
If I am playing alone, flying a Cargo shop with limited defensive capabilities - I fly solo. Why bother taking the risk when running fortification or Prep? There is nothing to gain from it.
In the last Powerplay cycle, we had one hell of a prep race with Mahon for a contested system bubble (I'm with Hudson). We had battle advantage flying trade conda's, and ran in open hoping we would run into them to destroy their ships. Unfortunately for us, we did not see a single one of their ships. We assume they were all running cutters in private group. I would assume their reason for doing so was they knew they could not stand up to us in a fight, and there was no advantage for them in taking that risk and attempting to do so. We ended up loosing that prep war since we could not keep up with the amount of prep they were able to achieve with those cutters.
So to make a long story short. It's a risk / reward mechanic currently lacking and affecting powerplay balance. If you are a Combat pilot flying in open, you have less risk than a Cargo pilot. As a Combat pilot you have a higher reward potential (kill enemy commanders to erase merits, fortification supplies, prep nominations). As a Cargo pilot, you have absolutely no reward for flying in open.