But since the archer can cast level 8-9 spells, the mage still retains some advantage, albeit arguably it's just a small one (I think you can get good damaging level 1-7 spells, especially with skill potions.. + all the 'utility' spells are there). In return the archer gets *much* higher combat skills, better weapons, and much better armor and hit points.
This is all true. But I found that by the time I got my archer's best spells, they weren't really that useful anymore. It was usually better to have the archer attack than cast anything, because at that point I was having my mage blast away with some seriously powerful magic that far outclassed the earlier spells.
To me, the spells my archer got were almost just a convenience feature. I could use my archer to cast some of the weaker spells if necessary so my mage was free to throw around something more powerful.
It's true that there are some good spells at levels 6 and 7, but by the time my archer got those it was already really late in the game. The delay in acquiring spells for the archer really means they are outdated by the time the archer is able to cast them. If the archer got the spells at the same level as the mage, then I would agree with you that they're a better class to pick, but getting more powerful spells as soon as possible is a much bigger advantage in my opinion.