You still have to cut a precise nut behind it. But if you want open strings to sound like a fretted one then so be it. Personally I like to have a nut and have a "different" sound to an open string for playing and wanting to make a big statement with an open string note as they seem to be louder and bigger when slammed or chugging along eighth notes with some fret noise there is nothing like it. Whatever is clever for you and makes more sense to you.
The nut shaping precision required is reduced by half as you only need to align the strings horizontally, and you can avoid having to fine tune the individual nut slot heights. In a headless design an "alignment" nut is not always needed. Here are a few thoughts:
1. In a headless design that uses a headpiece with slots or holes that align the strings an alignment nut is not needed.
2. When considering tonal consistency between open notes and fretted notes keeping the material of the bridge and zero position (e.g. nut, zero fret, etc.) provide a more consistent tone. If the bass is fretless a wooden nut will help to keep open strings sounding more consistent with "fretted" notes if this is the effect you are shooting for.
3. Many opinions exists on what the best relative height of the zero fret to other frets. Some argue that the zero fret should be higher than the other frets, and others feel all the frets should be the same height. This is a hotly debated topic.
I mostly build headless/fretless instruments, but I have used a zero fret successfully on several without an alignment nut in place. My
most recent build used an unslotted nut using the same material as the bridge saddle so that I could get a more consistent woody tone on the open strings.
Take everything I say with a block of salt as this, as with all things, falls into the "it depends" category. There are a lot of ways to skin this cat, and your results may vary due to a large variety of factors in play.