So borrowing summons from regular monsters in FF is ok, borrowing bosses is bad ? Makes sense.... Also summons and espers are totally different entities in ivalice, Espers are entities created by the gods, with half of them turning on them and in essence turning into the lucavi, while the other half remained loyal to the gods, which explains the duality of the zodiac stones, while they can be used to posses a host and summon the lucavi from the void, the zodiac stones can be used for good also, each stone has 2 entities, a light and a dark one. Summons are magical beings. As to why they dint use one set over the other ? Lore.
Every new final fantasy adds something to the series, if you had to follow the recipe of the very first FF, Final Fantasy would not be what it is today, with each new entry into the series, the universe expands, sometimes a new title will add a lot, and other times just a tiny bit, but is always evolving, by it by gameplay, story elements, summons, monsters, weapons, etc etc. Every game adds or takes something and adds upon it. For example summons where not what they are now, when the series started, they played a major role in FFIV, and then again in VI, an then they really where a part of the main story in IX, after that game they have been a primary part of the story as you can see in X, XI, XIII, and XIV.
There was no class balance in FFT, i mastered every job in chapter one and raped every single battle from the get go, also learning abilities from items.... That's FFIX system... To the letter, there is nothing changed, is the same system. And yes it is an easy mode FFT, but FFT was not hard, ogre battle was hard, Square hired yazmat to make an easy mode final fantasy version of ogre battle.
Yes FFTA series is a mediocre addition to the FFT series, because SE wanted it to be that way, that is why yazmat left.