My assumption has become more and more that taeguek forms are not intended as self-defense exercises but rather technical exercises.
I think that if the taeguek forms are intended to teach self-defense, they have a rather unrealistic viewpoint of what would actually happen.
Which is why I don't think taeguek forms are inherently about self-defense. I believe they are technical exercises for basic techniques, stances, self-control, self-awareness, balance, etc...
Bluekey has made some nice observations in this regard, and I think the jury should remain out till Simon O'Neil's forthcoming book on combat-effective bunkai for the Taegeuks comes out this spring, which (as I understand it) is the plan. O'Neil's
Combat TKD newletters, and his 2006 article in
Taekondo Times (absolutely the best article I've ever read there) are really about applications inherent in the taegeuks, and his analyses are both plausible and scarily effective/realistic. I don't recall him discussing the actual combat intentions behind the simultaneous punch/kick movements there, but something about that may well be in the book when it appears.
One big problem is that it's not completely clear just what the combat interpretation of the
kick part of some of these movements is supposed to be. I've mentioned elsewhere that in the SMK version of Eunbi, derived from Empi, the full-extension mid/high front snap kicks correspond to knee strikes in Empi, whose realistic bunkai indicate that such knee strikes are setups for a hold+groin-targeted punch combination (ouch!!:uhohh

. If you turn them into high front snap kicks, the whole interpretation has to change along with the revision. This is one reason why I've felt for a long time that to get 'clean' applications for many of the kicking moves in the TKD forms, we need to get a better picture of the original karate sequences which supplied the 'prototype' for these sequences in the Korean forms (as is the case with almost everything in the Palgwes, and quite a bit of the Taegeuks, as O'Neil takes some pains to demonstrate). Seeing just what the 'source' movements were, in connection with the kicking techs, would give us a better idea of how much was driven by clear combat intention, and how much by a kind of stylistic 'über-rule', as in the Empi ---> Eunbi development.
Upnorth's view of things is fairly bleak on the degree to which a coherent combat scenario was involved, and I'm personally inclined to go with his take on this, in spite of the fact that, e.g.,
foot2face has produced some very interesting 'reverse-engineered' bunkai for some of these sequences.
In a nutshell, it seems to me that the answer to the OP question may well be that if the coordinated kick/punch movement represents the result of what I've called a J-to-K 'translation' rule based strictly on certain
stylistic requirements ('all leg techs are kicks, and all kicks are mid-to-high'), there
is no well-thought-out rationale for the simultaneous kick/punch movement...