As a part of the original MKH range of mics, the 40 is part of a range that has been in service for a very long time, and while it's not the most used of the set it still has a part to play in certain situations. Its cardioid pattern makes it very forgiving in fast close dialogue scenes, and simultaneously makes it sound very natural and open in the right acoustic space. As a boom mic to be chosen against say a MKH 60 and MKH 50 it has a lower key and less frequent part to play.
One of my favourite uses for this mic is in an MS setup paired with the MKH 30 fig. 8 mic. With this combo it's possible to get incredibly realistic stereo images from what is practically a point source. With the matched capsules and MS rig of MKH 40 and 30 sounds uncannily realistic, with a neutral frequency response.
The MKH 40 has low cut and a 10dB pad switches on the body of the mic, so you don't need to use in-line modules as you might with say the Schoeps CMC+MK range of modular mics.
A great sounding mic, but not necessarily the first choice for the boom.