[Murph]

The reason for wanting one of these compilations (HSFX or UP) is that most of the material in them has not only been collected in one compatible install, they have also been re-coded to .sfs format, which makes for a much faster loading, smoother playing game than was possible with UI.
Most players nowadays, both online and offline are using one or both of these packs as a starting point for that reason.
Adding new maps to these packs is not any harder than with UI, it just uses a slightly different method.