I actually like AD less and less but that's probably because I play him wrong/too bad.

However, once I believe I had a live MP with some friends and I was able to push the AD hero up to the spell "call beast horde". The spell can be banned which doesn't make him too useful in fights against human players/enemy factions but against all independent units, it's pure gold. If somehow your enemy leader is not having any cp left, it's also incredible in a fight against them. If you're lucky and you spawn one or 2 spiders that can web in round 1, it can change the outcome of a fight completely.

Still it's hard to get there, because I do not find the other combat spells of archdruid too appealing, they are very situational.

The debate around AD can be very interesting!
In my opinion it's all about timings and other factors, such as what is your leaders class?
For a Rogue, Sorceror or Dreadnought I always go for the army leader skills with the AD, as the early skills favours healing (at lvl 3 you get both natural healer and nourishing meal) and once you pick natural healer, you might as well go for the other army leading abilities? Also, if you play Draconian Warm at Night might be beneficial and as an High Elf, you might want to go for Natural Immunity. 

But as a general build, I prefer to take Nourishing meal, Free movement, Druidry 1-2 so you have 35cp, I wont bother going higher as most games wont go so far, but in a large map beast horde might be a viable option. I always take Call Lightning, if nothing else it's a great way to make the defenders attack without wasting mana, and you can get the additional stun as a bonus! Regrowth is more tricky, as you say, you need to have a melee oriented druid that have a lot of hp, which is quite rare. That's 10 points potentially spent better elsewhere. Call Ancestral Spirits can be very useful to save the battle or to drain movement points of a hard target, so I usually take that as well. Haven't experimented so much with twisting roots in high level sites, but that might be a very good option as well! Probably a under valued strategy that I will try!

I don't exactly know what the "classical AD" picking order is, but I only take swimming and free movement with AD if I plan to make him leader of his own little fast stack. If I do that I really like to take the leadership abilities too, he can give spirit, frost and blight protection, so if you let him lead a group of phantom warriors they have min. 40% protection against everything (with their inherent shock and phys. protection) except fire. But I often use him as secondary hero in my main stack as healer and caster and then I don't pick the free movement and swimming upgrades because he'll be fast enough anyways. Then I pick Druidry I and II and hornet swarm but I'm PBEM noob and in Live MP my druids rarely get to a level where they can pick beast horde or lightning and the like :/ Twisting roots is good for sure, just I doubt the AI uses it smart in autocombat...

When an ally is adjacent and enter battle with you, I think the AI controls it but otherwise you play normally. But I believe it actually never occurred to me.

New debate: caster AD vs "classical AD"
How many CP do you take for your AD in general and why? Which spells do you choose? Otherwise, what build do you prefer?

I used to always take Regrowth and Call Lightning but I'm starting to think it's often a bad choice. Regrowth's benefits are actually limited if you don't take a lot of HP and with max level 13, you can't get that many, except if you are lucky and have good items (Jack's Gift and the +15 HP Shield for instance). In auto, the AD can be killed in 1 round without being able to regrow. As of Call Lightning, it can also be used not that well in auto and it's also costly.
I almost always take Swimming and Free Movement, which allow some nasty moves.

I'm starting to appreciate more and more AD's spells, like Call Ancestral Spirit and Twisting Root, which can be very good for clearing.
Call Beast Horde look nice too for auto, but not sure.

Hei do you know what happens if an ally and I are standing next to independent units and I attack, will my allied forces be taken over by the AI? will the whole fight be autocombat? Will the I take control over his allied forces for the fight?

Hi people. Sorry i was sick for 3-4 days. Food poisoning i guess. I am better now. I did my turn. As soon as i feel better i will read the comments.