Why don't we ever see characters?
Shouldn't covers represent what the story's about?
Although the saying is, "You can't judge a book by its cover", people often do. The purpose of a book cover, from a markerting standpoint, is to catch the browser's attention. Research has shown if you can make a person stop to examine a cover more closely, then they are more likely to pick it up, turn it over and read the blurb. If they have the book in their hand long enough, you are more likely to make a sale. (There are actual time & motions studies on this.)
With ST, you're trying to appeal to any number of the wide variety of ST fans and collecors, plus non-fanatical potential fans with a mild interest in ST. There are those who like and demand photo realistic covers (ie. that's why IDW often uses stills from the shows as chase covers, and why comics sold in bookshops often have magazine style covers). There are fans who want more artistic renderings of characters and guest aliens. There are tech-minded fans who want starships or battle scenes.
So Pocket offers a range. If a style of cover helps to sell lots of a particular book, then that style may be emulated in future books. For a long time in the 80s and 90s, Pocket came under fire from some fans for so many covers with "floating heads". Changing art styles, and the possibilities opened up by Photoshop etc, make the possibilities much wider today.
Consider though: if an artist paints a cover featuring Picard, and Patrick Stewart hates the painting, it must be redone. If an artist uses Photoshop and Patrick Stewart hates the image used (unlikely 'cos it's already likely to be an approved publicity still), it can easily be switched in Photoshop; not as time-consuming or costly. If a starship or planet scene (or Calhoun's ubiquitous sword) is used, it only has to suit Pocket and CBS.
In these uncertain economic times, starship covers (sometimes even recycled from past "Ship of the Line" ST calendars) are very practical, but still appeal to a large group of book buyers.
