I guess I'm the oddball here. I ride out into the pasture of my other horses all the time. I wouldn't advise lunging a horse in a pasture full of horses because a loose horse darting between you and yours could easily get tangled in the lunge line and cause a huge wreck.
However, riding in a pasture full of your horse's friends is a great way to gauge the quality of training your horse has. If you can't control him in the pasture, then do you really expect him to behave perfectly at a show or on a trail ride with other horses around? I'll often go out and "herd" my pasture horses on a young/green horse because it helps them learn that they need to pay attention to what I am asking instead of what all the other horses are doing.
As for the loose horses in the pasture, that is a great way to guage their training as well. The way all mine are trained, if I raise my hand and smooch to them, that means get away from me NOW. Works just as well from the back of another horse as it does from the ground...and if they don't listen, I carry a length of rope I can use to "encourage" them to move away from me.