Not sure if it's been mentioned in the thread so far as I skipped past all the noise about historical accuracy. But when I was first having trouble using couched lance to deliver strikes, someone recommended that I change to first-person view. (Default key is R, I think.) this made it enormously easier to put the lance tip into my target.
there are a variety of good suggestions I've seen people make about changes to lances, but giving us a "stab-cam" view over the shoulder, as seen in a few mods, would go a long way towards making couched lance strikes more user-friendly.
As to whether or not they're actually useful compared to other weapons, I'll leave that debate alone. I personally use it to seek out the enemy's heavy cav after I've given my troops the first set of orders, on the thought that top-tier nobles have the heaviest armor. So the high damage of a good couched lance blow might actually be the difference in 1-hit kills vs a need to reattack, and a quick kill on a leader might sap their morale before my main body even gets engaged. Then again, I don't actually know if the morale system is so sophisticated at the moment.
Whether that fits your playstyle or conception of the game, I'll leave up to you. But if you do want to use couched lances and you're having trouble landing the blow against isolated targets, just try switching to first-person camera as you get lined up.