Another reason for making melee weapons an option is Rule of Cool (not commenting on the relative cool of melee vs ranged, so much as the fact that sufficient people will find the idea of RPing a melee weapon expert cool to want that option). And replayability / flexibility - it's nice if you can try different modes of dealing with combat situations, either with one flexible character or two different characters in two different games within one system.

I can't really comment on the mechanical implications in most modern-world systems, however. To my mind, a modern-world system should cover rules for unarmed, improvised-weapon, proper melee-weapon, and ranged-weapon combat, with everything else being gravy. Unarmed should be mechanically inferior to the others unless you're going for very high cinematics, in which case, fine, your kung fu expert can somehow hit as hard as an AK47, but proper melee weapons should be marginally superior to ranged weapons at close quarters (less so for shotguns or sidearms than for rifles, but you still have to aim and shoot, rather than just swing/stab).