I disagree with pretty much everything you wrote.
And I tended not, in DA2, to spend points on upgrading abilities,, because there was often little direct benefit
That's wrong, the benefit is actually huge most of the time. All rogue upgrades for heavy hitters increase the solo damage a lot to the point that some skills (like explosive strike) are very weak before the upgrade. Better keep the chain 20% crit chance than break it to do spike damage that doesn't even oneshot a normal mob. There are very few upgrades that only worth it for ccc and that's mainly mage which has tree masteries that increase the damage of all the spells anyway so the result is mage becoming very strong solo character but actually having to get some levels first unlike DAO.
Warrior for example doesn't even need cross class assistance to be effective. Instead he can get cleave upgrade and add a stagger rng that when applied you can have a party mage do a chain lightning for some extra damage. It's nice to have, but the 5 second duration increase is more damage than a super powered chain lightning hit.
But you are right in the sense that DA2 has a lot of fillers.
So I didn't use Spirit Healer in DA2, because that character then can't do damage.
I don't see the same restrictions that you see for spirit healer. The main playstyle of a mage in DA2 is to blow his load and then stick to autoattacking and kiting till he can do it again. Keeping healing aura active from the start of the fight is the less optimal way to play the build. And the cooldown is a mere 10 seconds for the more prolonged fights. It's not spammable but it's not restrictive either. The inherent problem with that specialization is that it doesn't scale and doesn't provide any control, not that it doesn't allow you to cast spells. The 30% sustain cost is enough to create that problem by itself and if you try the obvious workaround you make an obviously weaker character, that being stacking willpower.
Now as far as "self sufficient" goes, I don't even see the point on this specialization. Potions are way more effective than any exclusive spirit healer spell for the main character in both games.
Also I don't really understand your problem with precise striking. Unless you have a specific encounter in mind where activating precise striking doesn't allow you to sidestep the boss efficiently or something other abstract thing like that, your damage increases. Plus if I recall right, in case you have a mage with haste in the party, you can't stack momentum+haste because of the well known attack speed bug, but precise striking allows it because it decreases attack speed enough to not trigger the bug. That's boring technical talk, but you want to tell me that this has anything to do with self sufficiency and versatility? It sounds to me like the bias I described about "how things should be". Counter intuitive doesn't mean less versatile.
Generally speaking I'm really impressed by how DA2 handled talents and specializations. If the enemies had the same level of quality and care the game would have a lot more replayability but they took many shortcuts and when you beat the game once, all subsequent runs are easy.