very useful, when you fight AoE spam happy mobs-especially with high magic resist and/or that healed by x during y garbage-pet parties can be a godsend. Parties generally need a mix of physical and magic damage, unless they are being formed for only 1 fight. The especially useful part is that even if you die when you get up your pet is usually still at full strength. You can stay out of AoE range and still issue your commands (excepting drg) and hopefully get new pets off to keep them mob from killing you when it comes looking. Summoner is actually better than beast at this because they don't have that long recall timer, which is why pup is ok, but you have to be cautious.
Seriously, believe it or not there generally is more than 1 way of getting things done in game. I've actually seen Shinryu beat by a party with a paladin tank without a brew (unfortunately it was a nin weap for the red !! proc, which we didn't have). Be open to new ideas, you might just like them.