I have not played Final Fantasy games beyond VI extensively, but anyhow…
Final Fantasy III: Generally, I found the black wizard class, and its superior version, the warlock, more useful than the summoning classes. I never used the conjuror, and I tried the summoner briefly to see what it was like, although in the end I just used the black wizard and shaman. I only used the Bahamut summon (Why bother waste a spell of the same level on Leviathan or Odin, when Leviathan inflicts less damage and Odin scarcely slew any foes for me?) when I got the sage class at the end of the game.
Final Fantasy IV: I probably used summons most extensively in this game. First Chocobo, then Titan (earth seems damage almost anything in the middle of the game), then Leviathan, and lastly Bahamut. I never bothered getting the summons acquired by winning battles, though (goblin, cockatrice, bomb, mind flayer).
Final Fantasy V: I used summons fairly commonly here, also, although not quite as commonly as in IV. Towards the end of the game, however, I tended to use more physical attacks (X-Fight whilst wielding two weapons). I loved the Golem summon, too; it was very useful in battles against the Sealed and Exdeath (first battle).
Final Fantasy VI: I rarely used the summons in this game except for Ragnarok and to a degree Bahamut (although I tend to get the Ragnarok sword rather than the esper), mainly because I tend to be somewhat conservative with using my party’s MP. The Crusader summon looks cool, but I basically never ever use any attacks which damage both enemies and allies. I did not use the Golem summon quite as often here, but it was still quite useful. On the whole, though, although many summons looked nice, I found that they scarcely worked as well as I thought that they might.
Final Fantasy VII: I have not played this game that much, but the summons looked decent here. I agree that the Knights of the Round do seem overpowered.
Final Fantasy VIII: Because I was introduced to this game by a fellow who made heavy use of summons, I tended to use them somewhat often, especially early in the game.
Is declining usefulness of summons a trend? It may very well be, but it does not seem too worrisome to me. Some games just favour the summons more. I guess that that makes for some variation in battle tactics in the different games, which to me is a bit of spice.
Actually, I remember once making a thread about how the Final Fantasy series was debasing the metal mithril, fearing a worrisome trend myself.