I'm willing to bet that 3 out of 4 built or being built models with split fuselages for engine display, are always closed up. It would seem to me that the R&D, funds, and time spent on those engines, which are usually more fantasy then reality, would be better spent on increased and more accurate visible details. GWH fantastic F-15C is a perfect example. Both engines are highly detailed, but that detail is quite questionable, and they'll end up inside a closed up fuselage where there isn't even the option of displaying them. Why in the world would GWH waist those resources that way? Beats me.
One other issue has surfaces on the KH F-86D, and that's the leading edge slats. They're molded as separate pieces that slide along molded brackets as the real aircraft did. The issue is that there is a lip/step from the front of the wing that the slat would butt up against while in the closed position. In real life, the back of the slat is tapered to point and slides up to butt against the wing, but there is no real step as molded. If you scale it out, it's quite a steep step. In reality it's hardly noticeable.
Since today's kits have much sharper trailing edges, the step issue isn't just to compensate for molding limitations. I would gather, as other have said, that it's to aid modelers in closing the slats up if they want to so they don't end up with a nasty seam that would be rather difficult to deal with.
Going back to the real aircraft, the slats are automatically dropped or raised by air speed and air pressure, so that when parked, the slats are in the down position. In every photograph I've seen, that's how they are. 90%+ modelers will show them that way. So wouldn't it be more prudent to follow the actual aircraft design, rather then the option they've chosen?
Joel