Worlds Without Number introduces a number of modifications to the basic B/X system. Many of which are really great, while others are rather puzzling.
Among the later ones is rolling the hit points for mages and warriors. Mages rolls their hit points not on a d4, but on a d6-1. Similarly, warriors roll a d6+2 instead of a d10. The total averages are completely the same, but this changes the odds for extreme values.
Mages have a chance to roll a 0. To that you add the Constitution modifier, and if the total is still 0 or lower, you still get 1 hp for that level, as you see in basically all versions. They also have a chance to roll a 5, which isn’t possible when rolling 1d4.
For warriors, it works the opposite way. For them, the range of 1 to 10 is reduced to a spread of 3 to 8. They have a reduced chance to get very low hp or very high hp.
Now one could say that for both mages and warriors, these changes to the spread cancel each other out. And the average does indeed stay the same. But what we get is that extreme results become more common for mages, and less common for warriors. The important thing here is that warriors can much more afford very low hit points than mages do. A warrior with low hit points still has a somewhat decent cushion to survive a blow or two. A mage with low hit points can’t survive anything. Having very low hit points is more bad than having very high hit points is good. So as I see it, this change makes things harder on mages than on warriors. Who also get bumped up to d10 equivalents instead of getting a d8 equivalent. (Experts got also bumped up from a d4 for thieves to a d6.) Do we really want to give warriors increased survivability over mages?
The other thing is a very simple statistical phenomenon called the Law of Large Numbers. The larger your sample of numbers you have, the more likely is it to be close to average. If you get large amounts of random numbers, it becomes more and more likely that the high numbers will cancel out the low numbers. If you have only two or three random numbers, the chance that you get all very high or all very low becomes much more probable. Once you get to 10th level, all characters of a class (with the same Constitution modifier) are going to have pretty similar hit points with only few characters being notable outliers. This means that the risk of getting very low hit points is much greater for low level characters than high level characters. Do we really want to have increased risk for low level characters?
As I see it, the move to have all classes roll a d6 with a modifier for hit points really only hurts low level mages the most by increasing their risk of being extremely fragile while increasing survivability for everyone else. What’s the point of that? This really seems like an awful change. That’s definitely something I’ll be changing back for my games.
At first I thought the change to only using d6 for hit point was because of the dual-classing mechanic, and it could be possible that this is where the whole idea came from. But the way dual-classing works now, you can absolutely replace those with d4s and d10s.