I only play open to try and make friends. Otherwise I stick to solo. No reason to encounter Dbags trying to hustle me for my cargo. Or the obvious griefer just out to blow up my ship for a cheap laugh at my expense.
And thats the main problem. The insurance costs are a real problem for open play... As a trader I don't mind loosing a Cobra when I am trading rares because of the moderate rebuy costs. But the loss of a Type 9 (without any upgrades) costs you already 4 million (5% insurance as a normal buyer of the game). With upgrades your at 5 - 6 million, if not higher. Many players don't risk loosing such a ship and the whole cargo (profit) on top. They are trading in solo mode and go to open play when they want to do bounty hunting with their viper. The players trading in open play on the other hand are more often a victim of the pirates, because their number decreases. They get frustated and play in solo mode when their first ship has been destroyed by an idiot who shoots at them on sight and so on and so on... At some point, only a few players trade in open play and the universe feels empty.
So why not decreasing insurance costs for trading/transport vessels (Type 6 / Type7 / Type 9)? Thus many traders will risk trading in open play because they cannot loose that much money.