Laura...Our cat is 14 or so and appears very healthy - eats regularly his high quality food that is always out for him plus some snacks, poops at least daily etc. But he is getting very thin. Should we be concerned or make an effort to fatten him up?
Since you leave food out for your cat all the time and then feed your cat treats and your cat seems to be eating and drinking fine... take your cat to the vet. You never mentioned your cat's age. Age could have something to do with it here.
You don't want to leave food out for your cat all the time anyway. Normally they would gain too much weight just like anything else. You would just want to feed them more times during the day. Take your cat to the vet though to see what is going on. Only your vet can tell you.