A word on the pure archer - it most definitely is viable, and extremely powerful. My party consists of a front-line tank (human, natural warrior), a pure archer (nephil), and two mage/priests. The archer kills more than any of them. Why? Well, three reasons.
First, adlerauge, the incredibly powerful bow from canopy which does 1-9 per level plus some ridiculous bonuses, is comparable to the highest damage melee or pole weapons. Personally, I think it's rather unbalancing. Ranged weapons usually do less damage for a reason, which brings me to the next two points.
Second, unlike a tank, which has to put lots of skill points into endurance and defense and the like, the archer is free to be entirely offensive, since they should only very rarely be in harm's way in the first place. In this way they're much like magic users. Also, while a tank should probably have a couple points in either bows or thrown, so he doesn't waste a turn if the enemies are out of range, the archer does NOT need any points in melee or pole weapons, since these will do less damage than the ranged attack anyway, there's no restriction on point blank shots, and it's easy and cheap to buy lots of arrows often. Thus, the archer's attack level (dex/2 + bows + sharpshooter) should be considerably higher than the warrior's equivalent.
And third, the archer should have high dexterity (and therefore high initiative) and doesn't have to move much to attack. Therefore, he will almost always get a very high damage shot or two (or three) before the fight even really begins, and does the same every turn while being in relatively little danger. What this amounts to is being able to pick which of your enemies you want to fight least, and killing them before they even know what's happening. This makes the archer particularly indispensible as a mage-killer, since silencing their spells early is important, and mages usually hang back and are resistant to magic.