Having Baby Jad, I started thinking how fun it would be to have one in my player-owned house menagerie, and one in my bank to follow me around. Not sure if I want to get another 100 Zeal, but it's a possibility.
Thinking about pets, player owned houses, and menagerie started me wondering about the pets people choose to keep. The menagerie is useful in allowing a summoning obelisk, so as I'm porting in from my last Slayer assignment, I can charge up Summoning points, pray at the altar, and repair any Barrows armor before continuing on. It also has a pseudo-useful function in the pet feeder. If you've got a pet with you, a quick click on the feeder will feed him. That's convenient if you like to keep a pet with you in your travels. But it's not a necessity.
All that to say, I very much enjoyed building my family of pets. And I get a lot of pleasure every time I go in my house and see them roaming around. It may not be extremely functional, but with all the pets, my house is a lot more fun for me.