Back when I had only a couple, I have let my piggies roam free (always under my supervision) once I had made sure that they mastered the "come back into the shuttle" procedure.
I have spent many happy hours watching my piggies explore, snooze under a bush and build a race course through the hot border while I was gardening. Especially Minx and Mischief couldn't wait to go out; it was the highlight of their day.
My garden is also completely fenced in and I always went and checked for any holes rats and other animals may have made. Sadly, we had a series of winters when we got rats squatting in the garden, so that put an end to the roaming. I can't do it with so many piggies now; that was one of the compromises I had to make.
You still have to beware of poisonous plants, birds of prey, foxes, dogs that somebody has let roam lose and the neighbourhood cat; and you need to provide shelters dotted around and within easy reach - most piggies take time to get used to wide open spaces and will always need a safe place to run to if spooked by a sudden noise. You also need to develop a pick up ritual if you want to avoid the flight instinct kicking in with merry chases at the end.