This indeed should be made optional.

The way it helps is that you don't get the variance/noise from picking a light at random, but if you've got many lights then computing all of them is slow. For the render you show it's a very significant improvement though. If you've got a reasonable simple shot with a few lights and rely on the global illumination to bounce the light around, then this should be very helpful.

The partial solution to slowness would to have some way to quickly detect which lamps contribute and which don't. That would involve introducing a non-physical cutoff distance for lamps and creating a light tree if you really have many lights, that's an optimization that can be done at some later point.