I find your question somehow odd...
Regardless of MIMO, directional antennas are nowadays often used for a lot of purposes: an excellent example is mobile telephony, that happens to be most "hit" by the MIMO technology.
So provided that your question is about general reduction of noise from multipath, I can explain to you how MIMO and "normal" directive antennas work:
Once upon a time... lol, well this is a subject that people studied in the early days, when we still had TACS technology. In those days we used spatial diversity to receive copies of signal that could be analyzed in order to understand how to understand how the channel (air) worked and how multipath affected the desired signal (this was of course not "manually" made).
Things changed a bit when a new generation of antennas came up: those capable of having dipoles which received different polarizations (if you look inside a base-station array antenna, you will find out that it's mostly made of couples - or the equivalent - of dipoles that are positioned to receive normally + and - 45° linear polarizations).
So we started to use polarization diversity to reduce channel noise (that is quite straightforward to understand once you read the above text).
MIMO works exactly in the same way (so I would say that it's not THAT big deal people are so enthusiast of): more antennas (even if basically attached) which in the best case have the same charateristics (otherwise you introduce new variables in the received signal) can achieve a noise reduction via spatial AND polarization diversity IF you use those antennas to do so. 441
MIMO is today mostly used to upgrade the bandwidth of the current cells so that more users are allowed to use the same band, or the same amount of users are allowed to have more mobile-internet speed.
So no, the observed signals must always come from sources inside the covered area.
About the directivity... "MIMO highly directional" doesn't quite sound right, since the technology itself has nothing to do with the antennas being directional. You could take 2 omnidirectional +/-45°Pol antennas, "put them togheter" (not correct, but gives the idea) and you could implement your MIMO anyway. 129