A. Looking at what you've seen druids do. A fairly common druid shape shifted into a bear, if you killed Kadga you'd see her as well shape shift, fought Nettie shapeshift. All these forms are from druids that can't help you, because their not strong enough. .....
Not going to quote all the text, but lol, here we go:
a. Well, again, healing and fighting are diff things, but surely you don't expect Halsin to single-handedly kill every goblin in the temple? If we are judging Minthara&co. by the same criterion, how come they're so weak that they can't succesfully attack the grove. EVEN with superior numbers. The way the battle plays out if you side with them is that YOU, the player char, get rid of everyone in the grove. Minthara and her gang contribute almost nothing ...which may be a gameplay thing more than anything else, (or maybe i was that lucky) but still... Not exactly an inspiring display of power, is it?
And Alright.. lets roll with the 'Halsin is a weakling'. Would it not be more lucrative to keep him around as opposed to keeping Minthara and co. around? You know, to ensure that your opponent/rival (in your example Halsin) is easier to eliminate if the need ever arises? Why keep the 3 goblin leader around if they are overall a bigger threat to you?
Would it not be easier to use Halsin, 'cash in' the favour?.. have him serve u?.. since you saved him and all?
b. They have to prove they're useful. OK, but why would you waste time and effort, if you DON'T KNOW if they're going to be useful at all? At that point you're operating under the assumprion that surely if i help them they help me. But why would they help you in return? Why would they not, being the evil guys, use you and then throw you away?
Again, since Halsin is a 'good guy' you can trust that he at least, is going to hold his end of the deal. Which means you have higher chances of getting what YOU want.
c. If you don't want Halsin's help or don't want to help him - fine. You know what you can do? Kill Minthara and talk to her corpse. She'll tell you where to go next. She'll point you to moonrise tower or whatever the name.
d. Evil has to be appealing, otherwise nobody would choose the evil path. If i have to do the same amount of work, or arguably MORE work, to be evil, then why would i? Why would anybody?
Unless as stated above they're 'delusional or suicide cultists'