I shall focus on D&D which seems to be where you (claim to) know the rules:
1) I don't need Wikipedia, because I have the actual manuals.
2) If they are not spellcasters, why in the hell does EVERY SINGLE D&D edition have a spell progression list for them? Because they are spellcasters.
Which is the main point. Spellcasting is not a role, it's an ability. A spellcaster casts spells. Let them be arcane or divine. Verbal or somatic. Offensive or defensive.
Wearing armor or not has little to do with the mechanic role's point. A wizard cannot wear armor because arcane spells have a somatic component, which is hindered by armor. That is why other systems have no problem with having armored wizards. What defines the role, it the actual spells you can cast.
Regarding roles and mechanics, even if its true that classes have main mechanical roles (i.e a rogue is a skillmonkey/DPS character; a fighter is a damage dealer and endurer, etc...), you can play your character in almost every way. For example, a wizard can cover every role:
Wizard with defensive role -> focus on abjuration
Wizard with offensive role -> focus on evocation
Wizard with contoller role -> focus on conjuration/transmutation
That is, what defines the "role" is the nature of the spell, its "school". And that is the reason why, even if a cleric is "defensive" (most of the spells' effects will focus on healing/buffing), you can spice it up with control spells from """necromancy""", for example.
Regarding the cleric,you can emphasise the combat aspect by becoming a warpriest when you meet the requirements. The game balances because you are less of a "pure" cleric, which is less of the traditional "defensive support" role.
Even if it won't the best in X, you can build a character who is decent at any reasonable role. (Note that it's just a drop in the ocean of character building, just a quick example)
It is true that the most simple of players will think as if it was WOW or a Diablo clone and will use the cleric for healing, the rogue for using a bow, the warrior for slashing and the mage for casting fireballs.
But, at least under my experience, the only edition with a tight limit in the roles is D&D 4, which nobody will take as a true roleplaying reference (nobody serious, anyway).