Because there's a delay at the start of both primary actions, switching between shooting and sword-swinging has a cost.  Whenever you switch, you lose a bit of attacking time.  This adds interest to the fundamental fighting action (even if the player isn't consciously aware of it, as I wasn't until I studied it just now).

There's another important limit on the player's actions, and this one I had noticed before.  With your sword, you're limited to three swings at a time, and there's a brief (under a second) cooldown afterwards before you can start shooting or moving again.  So if you haul out your max sword combo, you'll be vulnerable afterwards.  This is an important tradeoff that makes the "sword" decision more interesting for the player.  (With no limit or cooldown, you could just spam sword continuously and be mostly invincible.)

These are both good design elements that I will borrow for Ninja Tree and see how it works.