Don't quote me on this, but personally I tend to see "Dry Directivity" as some kind of "Wow!"-feature. :-) It makes the effect of those painstakingly assembled Instrument Profiles more obvious, but in general there are not so many scenarios where you will really need it.
I've explained the idea behind MIR's Dry/Wet handling in another forum message recently: -> http://community.vsl.co.at/forums/p/30894/197289.aspx#197289
In short, the "wet" component derived from MIR's multi impulse responses can be seen as the signal coming from the main microphone array, like it is used for most (if not all) orchestral recordings. The "dry" signal component represents spot microphones close to individual instruments or ensembles.
As long as "Dry Directivity" is off, these spot microphones will stay in their ideal position, even when you rotate an instrument on a stage. (An example: Imagine a spot-microphone in front of a trumpet player. This microphone is still in front of the player, even when you decided to have him play with his back turned towards the audience and the Main Microphone array. Miles Davis-style. ;-) ...)
As soon as "Dry Directivity" is activated, the spot microphone will behave like it has been
virtually
"nailed to the floor". The instrument can now turn away from it. The trumpet player from the example given above will have a spot microphone right behind his head when you rotate the Icon to 180°.
I think it is quite obvious that the spot-microphone will sound best in its ideal position in the majority of cases. :-)
Of course, "Dry Directivity" doesn't change the behaviour of an instrument in respect of the wet signal component created by the MIR engine.
HTH,