Downforce without the drag, or speed-dependent component of traditional wing downforce! So why does only McLaren use this relatively simple yet very effective technology in roadcars? Fans can be used to increase the effect of existing profiles for reducing pressure underbody, aid cooling and airflow through engine bay, as well as their own sucking car to the ground action...the only negatives i can think of are cost which is surely less than other relatively complex automotive endeavours and complexity which again compared to tech such as hybrid systems etc is much more simple. Plus what this system provides is unique, there is no other way to increase grip uniformly (not speed dependent) using aero and this method creates the least drag. Chapparal and Brabham both established this as an incredibly effective system in the 60s/70s, making this another old technology that lies in the dust, proven, just waiting for an effort from more automakers.
This pic and further info here