The issue is having a bullet do Heavy Melee damage and proc effects. Say you've saved for Lifestrike, Projectile Punch and Spirit Snatch. With one bullet, you could:
-Reload your gun + add extra ammo
-Apply 60% movement slow
-Steal 12% Spirit Resist
-Steal 26 Spirit Power
-Heal yourself by 55% of the bullet's damage + an additional 110hp
Not to mention, the bullet acts like a Heavy Melee attack, so a cross-map shot (that would normally do about 3 damage) would apply all these debuffs, an extra 80% melee damage on top of the added weapon damage bonuses applied by other weapon items, and doesn't account for damage falloff. All of this, behind a 12 second cooldown.
It is expensive to get all of the melee items ($11,250), but in the long run it'd always be worth getting since you'd do a ton of damage without any of the risks that come with melee builds.