No SMN casts physic either, but we still have it. As for the buff skill, there are ways to make it balanced with the others, and right now no one uses anything but sicfrit anyway. Yes, having 6 summons would be a pain to balance, and most likely every patch that rebalanced them would result in 1 or 2 that were going to be used over the others. And then the next patch would see which ones are in the spotlight change. That's better than 4 years of Garuda, and then a total swap to Ifrit.

Additionally, as you say, the pet with the highest DPS will be used. So when the people who run the numbers find out how much stronger one pet is than the others, you have numbers for how much the other pets need buffed to stay competitive.

As for buff skills, there are plenty of status effects that aren't being used much, particularly by SMNs. Ramuh might, for example, have lower DPS than Garuda but be able to apply a magic vulnerability debuff to boost raidwide damage. Obviously the number would need to be carefully watched so it didn't become too powerful, but that's what patches are for.