Originally Posted by
Dusk
They usually promote these adaptations when the TV shows are still running weekly. For example, the Castle book by Bendis appeared in one of the episode because it was released at the right time, and Castle wasn't a stranger to these type of promotions. Also, the Castle book was actually a comic adaptation of the Derrick Storm novel series, which was a series written by the character, Richard Castle, before the story in the show started, so it worked just as well as the Nikki Heat novel series.
The Dark Tower comic series was hyped to hell back when it was first announced. There were plenty of interviews, and they continued by adapting another book by Stephen King called "N". Those were proper adaptation, however, not TV tie-ins, like the Heroes comics were.
Revenge, Castle and Once Upon A Time are all shows that are airing on Disney's ABC channel, so aside from the fact that it's easier to deal with the licensing, I would suppose that there are promotions via TV ads. Or at the very least, ads on their respective websites. Besides, the fans of the shows, who are definitely the target audience, would be aware of any kind of comic adaptations.
Dexter is not a usual adaptation, in the sense that it was written by the series's original author, and thus the book took place in the novel's continuum, not the TV show's.
Personally, I feel that there is no benefit to doing a comic adaptation while the TV show is still running, unless your main intention is to promote the TV show. They are usually mini-series with only a handful of issues, and their messages are always, "if you wanna see more of these characters, check out the TV show". The only exception might be Doctor Who, which is always an ongoing series. Also, you will encounter tons of problems, ranging from continuity problems to not getting the character's voices right to having the comics be plot heavy with minimum character development. The only ones that were done right are the Buffy comics that was produced and written by Joss Whedon, the creator of the TV series, and the Smallville comic, which is written by Bryan Miller, who worked on the Smallville TV series. Both of these examples are actually continuation of the TV series after their series finale, and were/are written by their original writers, so they were/are easy to accept, even desired, by fans.
If you really don't want to read comics of the superheroes genre, there are plenty of other comics out there besides Marvel and DC. Seconds by Bryan Lee O'Malley just came out this month. Check with your comic store. If you want something that's more similar to what you find on TV, The Walking Dead is a good bet. You might spoil your TV experience, though. Fox's Sleepy Hollow is also getting a comic adaptation from BOOM! Studios, but I'm afraid it might read like a fan-fic. Technically, cartoons comics are also TV series adaptation, so I would recommend Avatar: The Last Airbender from Dark Horse comics, which is again written by the creators, and read just as well as the TV series was. Marvel has also announced 3 Star Wars series at the recent comic-con, whose writers include Jason Aaron, Kieron Gillen, and Mark Waid. Look forward to that.
tl;dr
Comic adaptation of TV series are just tie-ins. They are there to add to the lore of the TV series and usually, also to promote the TV show to comic fans, not to stand on their own. There are exceptions, like when the TV series has clearly ended. In those cases, the comic series are written with the TV show fans in mind.