Dark Sun themes were pre-essentials player crunch. It is certain as they were originally published - that I actually liked - are a dead concept and this is what themes will be in future. It would be awesome if my conclusion is wrong and the other themes DO have some DS like ones in them. I wouldn't mind at all themes that are more compatible with essentials classes and others that are more compatible with AEDU ones. As it is though, AEDU classes always seem to lose out on options when it comes to any decision about essentials compared to the original classes. Developing two different kinds would take more development time and I don't think that's likely to happen frankly - no matter how much I would love that to be the case.
Even with Dark Sun, they were already trying to juggle and work around the whole AEDU vs. psionic issue. And, they have sort of made a 'path' to give encounter powers to the characters with no leveled encounter powers via feats. So they could concievably put encounter attack powers back in to the concept of themes.
Personally though, utility powers are the easiest to design/balance probably, since you don't have to see "what would ANY class do with this power which uses their best attack stat ..." in each case. While essentials may have played a role, it may also be that, just like they only did racial utility powers, they felt that keeping it to your starting encounter power (which scales automatically), utilities and static class features it would be easier to design.
Also, it's only because of the Dark Sun themes that this problem is coming up. If these were the original themes, it wouldn't necessarily have occured to someone that a class feature being added over and above all the existing parts of a character is actually
denying them something. And it's a bit weird to say the only way something can support pre-Essential classe is to inentionally be unusable or less usable by post Essential classes. Making themes and feats and paragon paths, etc, available for everyone is probably better han making them niche. And of the themes, the power-swap powers are an optional element to an optional element.
IF someone takes the theme, they might take one of the powers, but only if it's better than, or significantly different than, something they get from their own class. With utilities, that's easy, as the range of possible utilities is vast, and 'generic' ones keep getting added (skills, racial, theme).
Or, it may simply be that it depends on the theme. A gladiator would learn attack techniques, while a wizard's apprentice or alchemist are more backgrounds that would reflect book learning and thust lean towards mostly towards utility ideas, etc.