You *can* fret directly onto your neck piece of wood. Notice Shane's fretless builds, he burns fret markers directly into the neck wood, and strings her up! Frets aren't going to weaken the wood. If you use a round over bit on a router to shape your neck, then put on a fretboard ... and 3/4" thick neck, with a 1/4" fretboard makes a big fat 1 inch thick neck! Not everyone likes baseball bats for necks.
No, it's not necessary but two or more pieces laminated together should be much less likely to bow or twist than a single piece of wood.
A fretboard is rarely a necessity on a 3 or 4 stringer, but along with rigidity, it presents options re action height, and much more, you can play with the contrast of different timbers etc, but overall ,it gives you the best chance of minimising distortion, and a better chance of sitting for a few months and just needing a tuning tweak to play, Shane's fretless builds are as you say fretless, with enough height it could be a banana, and Shane could make it sound ok, but it's nice to pick up a guitar that's been neglected for a few months, tweak the tuners 1/2 a step and play
Hi, couple of things not mentioned yet.
It depends on the timber used for the neck.
A fingerboard timber is normally denser than the neck and can provide provide stability and better holding power for frets.
Im not sure if adding a fingerboard adds more depth to the neck, making it clubby, because I would have to add that much more thickness to the neck timber, for stability.
And maybe less than firm fitting frets in their slots or, a soft neck timber, could allow the neck to compress under string pressure and bow up, over time.
Just things I weigh up before building.
I actually use both methods, cutting frets directly into the neck, and cutting into fret boards. I've had no problems with bowing or twisting thanks to laminating the neck. I think it's easier cutting accurately into a fret board using my jigs, however I love the look of the fretted neck.