It is certainly true that adding bulk to your helmet has some drawbacks in crashes and such, and consequently warrants some concern. In the incidence of a crash, solid, small objects that are fixed to a specific area of the helmet increase the distribution of force on mounting area, instead of spreading it away from the impact site.
However, my justification has always been this: a light can prevent just more crashes than not having a light will save you once you have crashed. Essentially, by using a head mounted light, you have more freely accessible light and can therefore be more aware of your surroundings. Equally, your surroundings will be more aware of you. These two facts combine to reduce the likelihood of crashing in the first place.
Also, I like to think that landing helmet first with a light attached is just like landing helmet first into a branch or small rock. These are also likely occurrences in MTB riding, so I would assume helmets are designed with at least some thought on this sort of situation. It mightn't be the case in reality, and I haven't confirmed this with standards, but hopefully saying this will give you peace of mind.
So yes, as far as personal experience has told me, a fixed light on your helmet can potentially do some harm in the event of a crash, but in the rest of the time spent riding, lights make helmets safer, and therefore makes riding safer. Seems like a good idea to me!