Elwood,those three keys are critical...to folks that play left hand bass, and want it to sound correct...76 key models go down to "E", same as low E on a bass guitar....
That's where octave shifting comes in handy.My Reason Bass Refill software has a minimum requirement of 61 keys...so in certain situations in the studio,the number of keys can be very critical and if I were a prolific piano player(particularly in a live situation),I would not even fuss over anything less than 88 keys-period.
The fact of the matter is,companies such as Korg,have to give equal consideration to studio musicians as well,because of how integral software is to their craft and in a lot of cases,space saving designs are a significant marketing demographic.
As recent as 10 years ago,I would not have ever considered anything less than a 88-key workstation keyboard,but since I made the transition to software recording and as my studio expanded,I developed a preference for more compact sized keyboards.
To my knowledge,there are no stand-alone midi controller keyboards in a 73 key size,so I am very pleased to see Korg make a 73 key midi controller/workstation.