[Anak]

@Jaden and @Terisa de morgan
I've changed my original comment, you're both right they are not necessarily shortcomings. So I changed it to: "The video clearly shows the output of the kepub format, which can not be changed by the user:", and added after "empty text line after each paragraph", that it does not happen with all kepubs.

• I didn't use the term "wasted space".
• I dind't say anything about a comparison of digital books with printed books (or even implied)

Originally Posted by Jaden

I like the header and footer

True, I do not like the header (in its current form) but I didn't my mention my dislike in my post.
I said the header and footer are relatively wide, caused by a huge margin and the position of the text on the screen.
Some (not all) readers have a header and/or footer too. But these are generally much smaller because the margins are smaller and the text position is closer to the edge of the screen. E.g. The Cybook Odyssey HD has an option to turn the headers and footers on or off (seperately! Header and footer are also smaller). The Icarus Sense has a full screen mode, In the non full screen mode small header and footer are shown.

Originally Posted by Jaden

Also the Kobo follows the original layout of the publisher (in ePubs at least). That's why you only get the blank line if the book is formatted that way. And that's why it's unfair to make it seem like you will always have blank lines after paragraphs which Kobo devices - it's not true.

Originally Posted by Terisa de morgan

Point two and three are not related specifically to format but formatting. If I send my epub with the extended driver as kepub, no aditional line appears there.

Yes, it is caused by formatting not the use of a different rendering engine. A conversion from epub to kepub with the KoboTouchExtended plugin goes well (Thanks, jgoguen!).
Yes, Kobo tries to follow the original layout but because Kobo adds some own code to the kepub to make sure all Kobo functions work correctly (font size, line height, annotations etc.). Some code functions as a wrapper around the original code and this code is a sort of a "catch all", which is understandable as it is to costly to do it on an individual basis or manually. This Kobo code can have a huge influence on the output (e.g. add white lines between paragraphs). But true not with every kepub.
Kobo probably decided to add code to the software (epub) to turn a epub into a kepub instead on the hardware side (device or software app) because it was easier to implement and cheaper to get the desired output.

If

• original Kobo kepub does contain blanc lines between paragraphs, and the
• original epub does have no blanc line between paragraphs, and the
• user generated kepub (epub conversion to kepub with the KTEP) does not contain a blanc line between paragraphs, then

the blanc line between paragraphs are caused by the Kobo Code.

Originally Posted by Jaden

…you should not compare different formats of digital books with each other - not if at least one of the readers allows you to use another format.

No, you don't compare different formats with each other you compare readers. And if possible with the same book or same paragraph. And use the default publishers layout of the book. So books should look nearly identical, any differences are caused by the reader (engine) or css overrides/changes.
Kobo does not control the local environment of an in shop product comparision of different ereaders. If the comparision is done with kepubs that have a blanc line between paragraphs the output looks quite different when compared to the other ereaders. And potential buyers may think it is odd or even that there is something wrong.
So a fair in shop comparison depends very strongly on which books (kepubs) are preloaded on the device. I consider this tricky for Kobo.
Most potential buyers probably only epub (but this may vary among countries) and are not aware that Kobo also has its own format kepub. And asuming that they have done their home work they go to a local shop to see it in action. Which device they will buy totally depends on if it looks "good", "right" or not compared to the other readers. Their "good" or "right" not yours or mine.

___________________________
This is the Kobo code I mentioned (the red code ("kobostylehacks") is the "catch all" code that may influence the original output):

<title>Book title</title>
  <link type="text/css" rel="stylesheet" media="all" href="../Styles/stylesheet.css"></link>

<!-- kobo-style -->
<link type="text/css" rel="stylesheet" href="css/kobo.css"></link>
<script type="text/javascript" src="js/kobo.js"></script>
<style type="text/css" id="kobostylehacks">div#book-inner p, div#book-inner div { font-size: 1.0em; } a { color: black; } a:link, a:visited, a:hover, a:active { color: blue; } div#book-inner * { margin-top: 0 !important; margin-bottom: 0 !important;}</style>
</head>

body>
<div id="book-columns"><div id="book-inner">
  <p class="p9"><span class="koboSpan" id="kobo.1.1">Chapter 2</span></span></p>

  <p class="p11"><span class="sz"><span class="koboSpan" id="kobo.2.1">Thursday  13 September, 07.03 A.M., Victoria Street</span></span></p>
</div>
</div>
</body>