Mode 3 type 2 charging is semifast and ranges between 3.7kW (16A single phase) to 22kW (32A three phases). The current i3 can charge single phase 32A (7.4 kW) and will fill a flat battery in about 3 hours. The new 2017 version will be able to charge three phases at 11 kW but since the battery is bigger it will still be about three hours to a full battery.
Until recently I have never used mode 3 charging. I have not even owned a cable. The reason is that I still find the standard charger to be sufficient for overnight charging and when I occasionally need to top the car up during long drives I do not find mode 3 fast enough and use CCS rapid charger at 50kW instead.
So why buy the EUR 250 mode 3 cable? Well there could be some reasons. In my case I need to drive in an area where there are no CCS chargers. I also see that some commuters with 100km or more one way may need to charge at a faster pace at work.
Also, there are many mode 3 charging stations still for free and if you have the time this could sometimes be preferred. Finally, there are many more type 2 outlets than CCS and they are often placed in parking garages and outside shopping malls. If you need to charge and shop a mode 3 top up may actually be more time efficient than first running your shopping errands and then do a separate CCS charge at an awkward location.
An advice for cable buyers is to buy a high quality cable but not better than you need since it will only be clumsy. The current i3 can only charge single phase so do not buy a cable for three phases. Some very brief research showed that the cable BMW is selling is reasonably priced. Still it is the most expensive cable I have ever purchased.