Well, as far as I have noticed, Summoning is more of an attack-all move. FF4 summons were more useful when you needed to attack the enemy with a spread out level 3 spell without wasting more MP. Remember that when a spell was casted to all targets (enemies/allies), some of its power was lost. So, a Fire 3 spell, for example, would behave similarly to a Fire 2 spell cast on a single area. However, not all Summons were target-all, or of an element that was available as a Black Magic spell.
FF5 summons followed a similar trend as FF4, with the exception of adding more Summons. Having a Summoner or somebody who could cast Summon Magic is often useful. Yet, the job system focus of the game slashed some of the Summons usefulness.
FF6 Espers, on the other hand, were more strategically inclined. Only being able to be casted once, they were rarely used. Yet, this game featured much more summons than in any other game, and most of the effects of such were of great use. Regen all, inviz all, image all, reflect all, Mute all, Muddle all, turn into items, allow everybody to jump…it made players think when to use them.
FF7 Summons are always useful. This game added more customization to Summoning, such as casting the spell twice, 4 times with lesser power, at the moment of dying, with more power, etc. Eventually, all summons except Knights of the Round lose their usefulness...but then again, most of the spells do lose their usefulness too.
FF8 GFs became way too useful. They could get some of the characters hits, and the Boost property of some summons, added more usefulness to them. Especially since magic was more needed for stat boosting and other stuff.
FF9 summons sadly lost much of their power to Vivi and his Black Magic, Steiner and his Sword Magic, and Quina with its Blue Magic. Ark was pretty cool, but that "send weapons instead of appearing" slashed most of their usefulness.
Now, FFX aeons...personal bodyguards designed to fight for Yuna, IMO. They became even more useful than ever, especially since even the low-power summons have their usefulness. Even after getting Anima and the Magus Sisters, summons such as Valefor remained useful. The elemental focus and the lust for overkilling added to their usefulness.
FFT summons play a useful role at certain parts of the game, becoming useful spells because of their range. Of course, level 3 and 4 spells still dwarf the summons power, and spellcasters weren't too useful in the long run anyways.
Finally, FFT:A summons…more power added, could be doublecasted, but only one race between all could use them. Then again, Black Magic could be used only by some races too. And the Totemas are way too overpowering…
Whew, that took a lot of time, but that covers up the differences. And about the Mythril power…I base myself upon the Chrono Cross version. In which other game do you get easily explained that Mythril is a byproduct of silver (Ag47, meaning 47 silver atoms per molecule). Well, DnD also covers up Mithril in its own way (lightweight, strong as steel), so being weaker than Gold is quite explainable. Then again, who’d use gold as an armor material, considering how scarce it is, and that its usefulness depends on adding silver and steel to make it stronger?
yeah, the Summons that didn’t do damge (directly), but helped your party instead were the only ones I used. phantom, Golem, and palidor- those are the onyl oens I ever used.