The damage of them has always been fine, unless of course you are trying to compare a low end summon to a high end spell. Do I expect Ifrit, who is usually one of the earliest summons gotten, to outpace Flare or Fire 3, which are usually gotten much later? No I don't. Ifrit is usually gotten before you have access to Fire 2 or about the same time, and his attacks are usually stronger than that, often estimated to be a Fire 2.5 because of where it falls. Reason you don't use it often, is usually because at that time MP is a very finite resource unless you have done tons of grinding and then it doesn't matter anyways.You have to compare them to what you would normally have at the same time you got them, not at the end when have everything. If you want to compare a summon to a spell like Flare, it had better be a late game summon.
As to the long animations, that is not really a good argument about them being useless or weak. Take Knights of the Round from FF7 for example, that thing had a horrendously long animation, however it was also one of the strongest attacks in that game. Could you likely do more damage than it did during that animation period? Quite likely, however outside of specific character's limit breaks there was no way to do more damage in a single action.