I'm not a big fan of nets(a pain in the neck), but if you are fishing light tippets, a net ends the fight much quicker, especialy if you have a buddy to scoop the fish up.
I'm not sure which is more harmful to fish, the loss of slime from being netted, or the risk of a long fight, thrashing on the beach, being manhandled, only to have the tippet break, leaving the fly in the lip.
I think that the worst thing that one can do to a fish is to drop it. Fish are not designed to deal with that kind of impact.
One cast can change your day...maybe your life.