I cannot think of a situation where you would get in trouble using a priv, where you would not get in trouble using the equivalent 4x the price full fare tickets, most people on here have either tried to avoid payment or have made mistakes with expired cards, occasionally because they have lost tickets or overslept, all of these instances could equally happen using a priv. I am sure that someone will now come up with such a situation but it would be incredibly rare.
Also you are more likely to be treated leniently by staff on the ground if you are using a priv.