Not only would a slow-motion capability allow the player to set fights at speeds they can keep up with and enjoy, but it would also allow for complex battle-dynamics based on a deep progression system. Targetting specific areas of the body, assessing the makeup of your enemy's gear, weapons, skills, etc. would provide for an excellent interactive fight sequence.
In some games I've had skills like disarm, or spells like corrode armor, block, bash ... but hardly ever used them because it just took too much precision timing and dexterity to find the right buttons to push in the right order. Once again, I'm a casual-playing old man of 50; I don't want to read and memorize a textbook just to understand all my abilities in a fight and then still not be able to use them effectively because my twitch reflex is slow or my connect to the internet isn't at T1 level.
Now imagine a skill tree that includes things like roundhouse, backside leg sweep, kneedrop throat-crush, corrode metal weapon, ignite armor (because you realize they're wearing leather or cloth, duck and thrust, duck and punch, front flip, back flip, drop and roll, throw a weapon, throw or kick dust, diffuse electricity, quench fire, evaporate water .... counter-spells that one could actually counter because the fight is going in slow motion.
Now, here's a further wrinkle: what if one of the areas of progression (lets remember that all progression in this game is 24/7, like EVE but more comprehensive) is reflexes, and training in reflexes allows you to alter an action in mid-execution, depending on other advancements in spellcasting, dexterity, agility, etc. Also, advancing one's speed, over time, creates a differential in the slow-motion progress between you and your enemy, so you can actually increase your combat speed in relation to the enemy's speed. The higher your training in these characteristics, the more sequences you can interrupt per battle, the slower the enemy moves in relationship to you, etc.
Slow motion combat sequences allow for all sorts of spectacular and complex battle interaction, and lets the player enjoy all of it from a cinematic perspective. It also increases the depth of options available in the various lines of training and advancement.