Both could have benefit.
Physiologically and metabolically speaking, it doesn´t make a whole lot of sense to do your VO2 max workouts at much faster than 5k pace (if you are a 15 min type person). But, races aren´t won by physiology alone. Runners need to practice running at goal pace as well as date pace (current race pace). It could break down like this.
pre season 5k workout= 20x400 at 72 with adequate recovery (What is adequate, I don´t know, 90 seconds, 2 min?....it depends on the athlete) The point of the workout being to practice running at goal pace without having the workout be overly taxing. During the pre season, the main training emphasis would be on volume and tempo running as well as general fitness, core, etc...). Granted, during this workout, the athlete won´t be doing too much to improve his VO2 max, but that isn´t the point of the workout.
Early season or mid season, provided there aren´t many races on the schedule, the same runner might do 6 or 7 x 1000 in 3:05 with 2 min recovery. If there are races in the schedule, he might do them with 3 min recovery.
Down toward the end of the season, the same guy might do 4 x 1000 @ 2:56 to 2:58 with 4 or 5 min recovery.