Reasons to collect more than one of any given pet:
- Some pets have abilities that benefit noticeably from multiple breeds (i.e. I run a P/P Flayer Youngling, some strategies call for a S/S).
- Some pets are pretty OP (and have unique movesets) that you may want multiples of it (i.e. Ikky, I know a few people that run 3 Ikky teams for PVE battles, as easymode).
- Some pets have multiple skins/sizes, so for someone that values that, you would need to have up to 3 of one pet (i.e. Parrot has 3 unique colors).
Reasons to not collect multiples of any given pet:
- As you noted the journal space is limited and it's impossible to have 3 of every pet, so as new pets come out you'll have to keep releasing extra pets to make room.
- If you're OCD it'll drive you crazy to see all those pets! And then there's the whole issue of stoning and leveling them!
![Razz :P](./images/smilies/icon_razz.gif)
And, honestly, are you really going to use them all?
I think most players do keep extras of pets that they find particularly useful. I collect for aesthetics so the duplicate pets I have are there to cover unique looks.