[MaxWilson]

Even though the mount is not explicitly modeled (in most cases at least), mounted commanders differ from regular commanders in several ways:

1.) They can't wear magic boots,
2.) Their melee encumbrance is not affected by the shields or armor they wear,
3.) They get an extra +3 to defense.

#2 and #3 are positive, #1 is negative. In some cases, the mount sticks around and fights for you after the rider is dead (most mounts are upkeep-free, i.e. goldcost per turn is 0, but their MR is usually pretty poor too). Some mounts stick around only for the duration of the battle (Serpent Cataphract mounts and those lobster rider guys that R'lyeh or Oceania has), others stick around until they get killed (Machakan spider-rider mounts).

Anyway, the slots on a pretender are sometimes an important consideration. One major difference between a lich and a master lich, for example, is that a master lich has 3 misc slots, which is sometimes the difference between needing to buy an extra level of magic in order to cast a certain spell (e.g. Arcane Nexus) vs. being able to plan to just stick on another booster. And as the manual points out, you don't want to plan to use a skull staff to cast Utterdark and then find that you picked a Monolith with no hands.

-Max