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Good points by the previous posters, I would also like to give my subjective view on the subject matter. The mechanics described below is how they affect my game experience.
For a 6 man party based RPG like this, I think it's important for the companions to be equally complete as your own main character.
By complete I mean:
1) If one of your companions is a mage, they need to have access to entire mage spellbook
2) Use the same stats
3) Use the same inventory slots
- etc.
1) 2):
When companions have access to the full skillset, they will feel like a real character. If not they will feel incomplete, handicaped,restrained in some degree. I wont care for them much and their oppinion will have slightly less credibility and weight (obviously their background story and the quality of the dialog will have the biggest influency on this).
Also it enchance the gameplay value; the longivity of one playthrough increases. Because I can change my focus to one of my 5 companion, when I do not feel like playing my main character. This is something I did alot in Baldurs Gate 1/2 etc.
Furtherly it cater players whom emphasises on customization and tweaking (more 'valves' to turn. adjust and play around with during a playthrough).
Obviously companions individual stats can be adjusted, not every companion have to feel the same.
3):
One of the very cool aspects of Icewin Dale, Baldurs Gate etc., it's easy to switch your items around within your party, you don't have to be frustrated with companion only items (I just want to point out, Im not referring to family items, I like those).
In BG most items had an uniq cool story, maybe I gathered rare part of an legendary items and finally got it forged. Elements like this makes you more attached your equipment, it becomes personal to you. Most BG players would probably recon they saved alot of uniq items in their Bag of Holding for sentimental value etc. So the point is: do not make specific companion slots or less slots, its nice to utilize as many 'personal' items as possible.
Companion cannot be stronger than your own main character?
As Bos_hyrbrid points out: "if the story calls for it, and it makes sense in the world/lore"; you team up with a legendary Paladin of the Xxxx, it will feel more convincing.
Companion might start out around your powerlevel, but as the story progress, the main character could gain extra features unattainable for companions, hence making you slowly stronger. In this scenario you will also feel progression, you have earned your power, satisfaction etc.
The main character always have the last word in dialog and discussion along your quest; you are in charge, you are the leader. Companion strength vs. main character strength doesn't have to be measured only in pure stat vs stat.