My current bass (a Skjold 6) has 18mm string spacing at the bridge, but tapers to a 2 nut width. It's very comfortable to play, and the taper of the neck keeps it from being too much of a plank, as Jimmy alluded to. It's probably my favorite bass of all that I've been through, ergonomically speaking, so I guess you'd say that I prefer a wider spacing, with the caveat of a greater taper towards the nut.
I've owned one other 6-string (made by STR), and it also had 18mm spacing, but the neck tapered to a larger nut (2.25, if I recall correctly). Combined with the profile of the neck (which was very thin front-to-back and relatively flat), the width of the neck made it uncomfortable to play for long periods, even with proper technique.
When it comes to extended-range basses, it seems as though small changes to any of these specifications can make a big difference to the end comfort of the player, and very few big manufacturers have been able to get it right, IMO. I haven't ever played an Alembic 6, but it seems as though my preferences would work better with one of the more modern body designs (Rogue, Europa, etc.). I can't imagine having the wider spacing and six strings on a Standard Point or one of its variations-the size of the body would make it unmanageable from an ergonomic perspective, especially given my experience with the (much) more compact body of my Skjold. Also, given the changes in my right-hand technique since owning my '75 SI, I don't think I could do what I do now with the classic spacing.