Depends on the design approach. And understanding that you might need one more than the other, and what group comps call for.
For example (looking at my chart), assuming SMN were given Leviathan-egi as a Debuff/DPS pet modeled around the fel hunter, Levi-egi would have an ability that debuffs target enemies (useful in PvP to remove stuff like protect, stoneskin, adlo and so on) and the ability to interrupt spellcasting (the way BRD can with Blunt Arrow and PLD can with Spirits Within). You know what happens if that were to be implemented? SMNs could become part of an interrupt rotation for raid bosses. If they were to introduce buffs for boss mobs that need to be dispelled, Leviathan-egi suddenly has a place during those encounters as well. It's obviously not going to make Leviathan-egi a must for every fight, but it would have its place.
Using another example, assuming SMN were given Ramuh-egi modeled around the succubus (the warlock CC pet), it'd have a single target lightning attack and Chaotic Strike (in this case as a channeled charm effect that lasts up to 30 seconds, with a reduced effect on player characters). So if there were any situations with adds that need to be controlled, SMN now has a reason to pull out Ramuh-egi, and would have the option to incapacitate one enemy player to peel for their healer in PvP. Again, not mandatory for every fight, but would have its use.