In a world with guns, using melee to it's full potential requires battlefield control. In other words, arranging things so that you get into melee range with your foe without getting turned into swiss cheese. In close quarters (most indoor environments, aside from large spaces like warehouses), this is actually easier than it sounds. Remember, most firefights take place between 10-20 feet (about 5m). So in those conditions, getting into melee range is not that difficult. The hard part is keeping your foe in melee range, and keeping his buddies from filling you with lead while you're chasing him.
Now, why would you go through all this work? Well, for one thing, melee is much quieter than firearms, as a rule. Yes, there is such a thing as a silencer, but this brings me to my second point: you can be in situations without guns, but you will never be without a melee weapon. That alone is enough reason that you should at least consider training in one melee skill. The third reason is that melee weapons are almost always legal, and can be brought more places than guns. Compare carrying a katana in Neo-Tokyo to carrying an assault rifle. Also compare bringing your favorite SMG to a fancy party to carrying in a gentleman's cane, reinforced to make a heavy club.
So really, everyone should at least know a little melee, for their own good. But the ones who focus on it should really only be the types most likely to get in those kinds of situations: street samurai, covert ops types, and some combat mages/adepts. Street sammies are obvious. Covert ops types tend to be in situations where guns, even suppressed guns, are a Bad Thing(tm), and a knife coated in Slab makes an infinitely quieter way of dealing with guards. And as for the Awakened types, being able to swing a sword or throw a punch actually goes very well with their Talent, especially if they have an Elemental aura/effect. That -half AP plus secondary effects makes a lot of difference when the Adept with Killing Hands throws a punch and lights you on fire.