What are the disadvantages of using importance sampling (MIS) for lamps?
I mean, apparently it converges faster, looks the same as when not using it, takes about the same time... Why not turn it on all the time?
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If a lamp is too dim to add any significant lighting to other objects (except for very close ones), including it in MIS just wastes computations for no real purpose. If you are using branched path tracing, it will sample all lights on every sample (with MIS). So you want to make sure it is only spending time doing samples that might return something other than 0 (or close to it). If you are using the simple progressive path tracing mode, it only samples one lamp per sample. So if a lamp won't contribute much, you don't want it to use MIS because it might steal the sample from a more useful lamp.
tl;dr: disabling MIS is a way to tell Cycles "this lamp is too dim to illuminate much of anything, don't waste your time trying".
I believe the disadvantage is that basically it converges faster than other light sources (such as glossy reflections and caustic refractions or MIS disabled light sources). What this will do is smooth out shadows MORE QUICKLY when it is enabled. If you want to capture the ultra-realistic shadow details than it is a good idea to turn MIS off.