A lot of my regular library families have pets already in them or sectioned off into their own additional library slot. If they are in with the whole family, they are always there. If separate, sometimes I add them and sometimes I don't.
I love how pets look (and pets have always been a crucial part to real families for me), but they feel so soulless to me on top of being able to do next to nothing with them. They are decorations taking up a family slot, so I have to make a call every time I play.