Ah okay. Yeah, I don't see a reason for doing that but it's your collection and you should do what you want with it. I know some people like having a wide variety of breeds, for example. I assume the pet journal might be increased at some point in WoD but currently, I believe the cap is still set at 1000 in beta.
Personally I find the pet journal already full enough as it is with 1 of each and a dozen or so duplicates for unique skins.