The question is not about could you equip your wizard with heavy armor and sword and put him frontline - I believe you could, almost any game having wizards allows you to do this. The question is - could you make your wizard efficient in this role, or in any other role different then casting damaging\controlling spells staying outside of the battle? Let's take D&D games again - stone skin, (improved) mage armor, mirror image, (greater) heroism, intuitive strike, blades of fire, displacement, tenser's transformation, iron body, a-lot-of-other-spells, feats, prestige classes... I can keep counting for ages, these things allow your mage to be very good in melee and even outtank your fighter, sure you have to cast these spells each time melee engagement starts, but you can make melee mage if you wish and he will not be completely underpowered to fighters\barbarians.And I would like to know the degree of PoE's character classes flexibility. Their ability to be efficient in different roles, not only in most straightforward one: fighter - tank, barbarian - melee aoe damage, rogues - melee single target burst damage and so on.