All right, the mischeviousness of this face is going to give me trouble sleeping at night...
Anyhoo: It was ironic because I read Keirsey first,
before going back and reading conventional MBTI theory and seeing that he had deviated somewhat from the standard way of breaking up the groups. I think I even saw some ST and SF groupings, rather than Keirsey's SP and SJ.
Here is just another small way to look at it, to springboard further discussion...
I don't think the conventional groups were quite as useful, at least in dealing with recognizing people from the outside (behaviorally). ST and NT overlap a great deal as do SF and NF (many of the latter Fe a lot -- I am thinking of the difficulty in sometimes distinguishing INFJ from ISFJ based solely on outer Fe behavior). And ST and NT can get very engineer-like/scientific in their interests and approaches.
I think Keirsey (as a theorist) really wanted to get back to motivations, and he saw that ST and SF people actually broke down into more defined groups along the SP and SJ lines. This is probably because S is more concrete, so their motivations and desires are played out in the observable physical realm -- SP wants the freedom to act (regardless of how that freedom is expressed) and SJ wants stability and structure in order to create a firm foundation for life.
N is more abstract in nature, so the evaluation (T/F) is much more important than implementation (as with the S's) because it determines what possibilities are actually observed.
So I guess (and I am making this up as I go), I can see a case being made for:
S = Implementation/Acting upon is the goal
N = Imagining/Envisioning is the goal
For S, SP vs SJ offers the larger distinction, and motivations are carried out in the physical realm.
For N, NF vs NT offers the larger distinction, and motivations are expressed in the intangible imagination realm.