I have a sprite pet I carry often, and a dark fairy I rarely carry, so she just sits in my backpack and doesn't get to do much. I realize it's just a computer game, but selling her in a shop feels wrong to me because she is like a little person. I wish we could set pet fairies free, or return them to Oriel's care. I bet there are other players who would enjoy returning pets they don't "use" to good-guy caretakers where they could be "happy."
Nothing wrong with collecting pets if that's what you enjoy, but I would rather find a pet a good home (not with the bad guy that dropped it), than own it and feel I'm neglecting it. If people want a lasting souvenir, there could be a badge showing that you both rescued the pet and found it a good forever home.