Fair point, but the majority of Discourse forums do not have an excessive number of categories so I don’t see why we should deny them the easiest option available just because drag&drop could be problematic in edge cases.
Furthermore, one of the biggest shortcomings of our current re-ordering interface is that it treats sub-categories as independent instead of being in fact a child of a parent category. Right now #howto:sysadmin can be position 3 while #howto is position 8. In my opinion sub-categories should be positioned as something like 3-1, 3-2 etc, inheriting the primary position of their parent category.
If only parent categories are shown by default in the re-ordering page, with sub-categories shown upon toggle, the average Discourse forum will have a very manageable number of categories to deal with.