Thirty books all with the same cover

Warning---in this post I am talking about a writer who writes erotica...well, that is the polite term for it.

So tonight, I decide to spend some time on Smashwords...because I had no desire to read my homework. While there I am checking out the various books that people are putting up. I had the adult filter off because I am curious about whether that is a viable money making choice.

(Feel free to blame my curiosity about possible money making options on the fact that part of my student loan has still not gone through yet, and the semester is about to wound down. And I thought some merchants paid out slow.)

While there, I noticed that one of the erotica writers (ok, let's be honest---it is pornography) has used the same cover for thirty of their books. The cover for Summer Suds, a sex romp that starts at a car wash is the same cover (with just the title changed) as Summer Camp Surprise, and the list goes on for twenty-eight more books. They do have a couple of other covers, including one that is used twice.

Nevertheless, I find myself wondering isn't there some rule of thumb that you want the covers of your book to be different from one another? Or does that just apply to actual physical books?

Now, I suspect that the only reason that this writer is even using a cover is that you must have a cover to get into the premium catalog (aka the rest of the distribution network). But seriously, are they so cheap that they are only willing to spend money on one "safe" cover? Looking at their regular website, I realized that they are willing to buy for the more racier, not approved by Apple style of covers (lots of naked flesh and sex acts if you are curious).

So are clothes really that expensive that more than one cover is out of their price range? Or does it really not matter if all your erotica titles have the same cover? And does this also apply to other types of ebooks?