Your mental health, ability to work, and happiness are all impacted pretty heavily by your diet. Lacking some nutrients can increase depression, lacking others can result in always feeling tired and sluggish, etc.
The reason I'm wary of soylent is not because I doubt that I could live on it (many people have lived for fairly significant durations on basically only soylent), but because I worry it would overall decrease my quality of life. Saving time eating is great, but it's not worth feeling like shit the rest of the time.