To be absolutely clear I'm only wondering about the necessity of having the boom.
You can have the self-tacking facility without the boom. It is easier to maintain optimal sailshape with the boom. Depending on the size and cut of your sails there can be a lot of turbulence in that slot. If you have the boom it tends to hold things a bit more steady, particularly in light to medium air. Takes some practise to get really smooth flow through the slot with two headsails. Oft times two will actually slow you down. Sometimes you can do better with one, particularly close-hauled.
Minor performance factor. You need to weigh the potentially increased speed against the pleasure of getting cracked across the ankles three or four times a year.