To be honest, I never understood Joe's over obsession with guitars... You don't need a gazillion guitars to play the blues. Just like Mark said, you can't play them all anyway. At least Mark really gets to use the unique properties of his instruments.
When you got a great guitar, you'll need another one. Then, 2 more... it's a fibonacci sequence.
I think Joe is one of that players that are playing a lot of his guitars on tours, recordings, etc. Every tour he is playing, at least, one or two different guitars from the previous tour. His vintage Telecasters, Strats, Les Pauls are always changing from tour to tour. Sometimes Joe lends some of his guitars to another players.
John McLaughlin once said that he don't like to see his guitars laying on cases, so he lends his 'unused' guitars for his friends knowing about they probably will return with some damage.
Bonamassa, always brings some different guitar to his records. So, his passion for guitars made them to appear little by little, tour by tour.
About that unique properties about MK's guitars.... i dont think that they have that uniqueness that people fantasies about.
When MK brings his "newest" Schecters to the stage, with his tapped-pickups. It was new for those who wasn't 'updated' about guitars at the time. OK, tapped-pickups was 'new' that time, but almost every Schecter (strat or tele) from late 70's until mid 90's has that kind of pickup.
The Pensa MK1 was unique... only by desing.
The Pensa MK2, a strat in a LP shape... look at the late 80's Valley Arts.
Maybe, in my opinion, the most unique was the red tele.
MK sig strat is a regular custom shop strat. Nothing really special or singular.