While technically you're correct, that you can't "stroke both sides at the same time"... if you've used a W.E. for any length of time, you'd see the difference. It's a bit more than just eliminating the "flip" time. (Think of having a KME where, as you moved one stone off the edge in a stroke... the next one was already headed down the edge... if that makes sense). And, especially in finishing a blade... where you alternate strokes to clean up the edge... the W.E. just does it better. You'll really see the difference if a blade needs some serious work. Not saying that any of this can't be done on the KME... but it is different. (In addition, sort of related, is the way stones are changed on the W.E. vs. the KME... a quick swap vs. stopping, unscrewing one and screwing in another). Like you, I think horizontal does give a bit better view... but it's easy enough to adapt.
Again, the "cheaper to add accessories" is a bit of a misnomer. It is cheaper because you add a "smaller amount" to the KME (one vs. two stones)... although overall it's a cheaper route, it's not as big a difference as it appears. I do think, in the case of some "hobby sharpeners". this may be the better way though, because it is a cheaper way to try stuff without having to buy 2 (or 4) of everything.... if that makes sense. (IOW, I can buy one 600g Chosera if I want to try it on the KME... really can't do that on the W.E. without some effort anyway). But if you plan on using stones/strops more than just a trial... the cost difference isn't as great in the long run.
Like I said, both will get a knife sharp... just a bit different way of getting there. The choice depends on what's important to the user in the differences.