I find Espers very useful sometimes.

When I fought Addremalech, I opted to use Quickenings to defeat him. After I failed at chaining enough attacks to kill him on about 3 different tries, I attempted to defeat him without Quickenings, and failed again. So then I summoned Mateus, had him die after he dealt a good 1/5 of Addremalech's HP. I fought, healed up, and then had Penelo summon Mateus again. After Mateus used his finishing blow, I summoned him once more with Penelo. When Mateus was dismissed the third time (Penelo died), Addremalech had about 2/3 of his HP left. I revived Penelo and used a long Quickening chain (17) and Addremalech had like a sliver of HP left, and I won.

So yes, Espers are useful to me. They only take up one MP bar (well, all the ones I have anyway) to use, whereas Quickenings take up all 3 (2/1). I can summon Mateus/Belias/Addremalech three times for every Quickening chain I can initiate, so I exhaust one summoner's MP, then have someone else start the Quickenings.