Part of the reason for that kind of advice is the luck factor. It's pretty strong in this new ebook market, because most writers (and even publishers) aren't really sure why certain books become mega hits and others sit largely unread. Thus the more books one has floating around the Kindleverse or any other ebookverse, the greater your chances of being discovered. I think it's also true that when readers discover and like one of your books, the chances are strong that they'll go on to buy your other titles. Writing a series dramatically increases this.
But I believe that a lot of authors have taken that conventional wisdom and completely turned it on its head. I know of some authors (none I know personally, so don't worry, I'm not talking about you!) who cram as many titles on their author shelves as fast as humanly possible. Many probably even use several pen names to accommodate the onslaught and I know several of them are making good money doing it, at least in the short term. Thus the ends seem to justify the means. If someone is in this business just to make a quick buck and doesn't care about the health of the industry as a whole or building a career that spans decades, then I suppose that approach sort of makes sense.
I've come to realize this first hand by reading reviews. I read a lot of reviews because I like to get an idea of what readers look for in the books they read and what pisses them off. Poor editing (typos and plot/character development) seem to be at the top of this list. And more often than not I noticed the worst offenders have dozens and dozens of books online, repackaged a million different ways. Some I'm told have even taken books they've already released and remade the cover and blurb and thrown them back on Amazon using a pen name.
The most egregious example I've heard of recently is one person (I won't even bother calling him an author) who has over 100,000 books on Amazon. Most of it is crap, probably illegally farmed off the internet and slapped online, but this guy's following the spammers golden rule. The more the merrier. It's the same numbers game I mentioned above but taken to the absolute extreme. Who needs quality when you can just flood the market right? And I'm worried that's the same slippery slope many self-published authors are inching toward (some as I've mentioned are already well on their way). It doesn't feel like spam if it's something you want and a product that's well made. Spammers sell crap in bulk. Authors shouldn't become spammers.
