They are completely 100% not necessary. The pickup sensing the string's vibration is where your signal comes from. A magnetic pickup doesn't need a soundhole to sense the string's vibration. A soundhole's original intent was to contribute to the acoustic sound produced by an instrument. On an acoustic instrument, the soundhole is a major contributor to the volume and tone, because it plays a role in how the body is able to move air, and that movement of air is what you hear. A soundhole (or two!) is arguably a requirement for an acoustic instrument, in the context that we are familiar with. An acoustic bass guitar without one will sound very wrong!
Can they potentially have an impact to the sound made by an instrument with a magnetic pickup? Yes, although I think the extent to which they make an impact is debatable. In a true hollow body with either braced top or a soundpost, a soundhole will impact the way the top vibrates, and soundholes that lead to a softer top will reduce sustain and top end in the bass's tone because the vibrating top is stealing energy that would otherwise remain in the string. But many "semi-hollow" basses have either a solid block in the middle or are otherwise reinforced to the point that they function more or less the same as a solid body bass, and the difference you'd get from adding a soundhole would probably not even be noticable. At any rate, things like string selection and pickup selection are going to make a much more significant contribution to the tone and sustain of the bass.
If I were in your shoes, I would choose based on personal preference in terms of look and feel, and not get hung up on the potential difference in tone or sustain. And, of course, trust what your luthier tells you if you have a good relationship with him! He will know what he is capable of influencing in a specific design.
