What exactly do you wants spirits to do? Dark Spirit to cast Death and Meteor and Light Spirit to cast Auto-Reraisega III and Ultima?
No, I don't think anyone actually believes that will make them useful. People don't use them for their spells now, and they already have pretty useful spells, even if a bit out-dated. However, being able to control their actions like those of avatars (even at the cost of spending mp on their spells, imo) would
make them more useful. You can give them all an irresistible death spell. It won't matter if they just insist on enfeebling the mob over and over.
They're used to siphon MP and to quickly summon without interruption to get away from mob aggro (god knows most SMN have such abyssal skill that they will get 100% interrupted for 1 point of damage from a level 1 rarab). They serve their purpose. They're not meant to be godly. They have their situational uses already.
The first part of your proposed purpose for them wasn't even implemented until several years after their release. The second part is completely useless to anybody for whom keeping a free/positive tick avatar out full time is a viable option. This has been easily done with Carbuncle for years (possibly since their release- I don't know when C. Mitts were added), and now all of the other avatars as well. I highly doubt that was all SE intended for them. If so they wouldn't have been given top-of-the-line magic (at 75 cap anyways).
They are sadly just another example of SE being unable to implement pet jobs properly. I like the idea of them operating like normal mages with normal spells, and I like the idea of SMN skill being the basis of their recast time. I don't even mind their high perp cost, because as I said earlier in the thread, they should be nearly free or more than free for most SMNs now anyways. I don't even mind them operating somewhat independently. I just wish their choices in spells to cast were a little smarter..
There are plenty of solutions to this. SE could throw in the towel and redesign them to be like avatars, where the SMN spends mp to direct it to use a certain spell. SE could tweak their AI themselves. SE could let the SMN set the rules for the spirit's AI in a way that's similar to BLU spell setting (for example, the ability to disable certain spells, or to change which spells they make a priority in what situations).
As for all SMNs having low SMN skill, it is by and large the hardest skill I have ever had the misfortune of trying to level. I can't blame other SMNs for not being capped. Old school SMNs were trapped in the main healing slot and barely used their summoning magic leveling up. New age SMNs have let their skills rot in Abyssea with everyone else's skills. Aside from that, it's just a pain to skill up anyways. If they hadn't added skill ups to bloodpacts, my skill would be pretty embarrassing.