Some of my favorite people online are the ones who don’t shout about how great they are; they shout about how great others are. They use their own blogs to promote friends who have published books. They have others guest host on their blogs without being afraid of losing their followers to the guest host. They understand how it works.
People are more receptive to those who aren’t always tooting their own horn. Nothing makes others turn the other way faster than a person who never makes an appearance except when he’s selling something . . . even if that “something” happens to be himself. No interaction, no give-and-take conversation, just, “Buy this/I did this/look at this,” and then whoosh! He’s gone.
There are millions upon millions of readers in this world. Their tastes vary. Just because I’ve read Robert Heinlein doesn’t mean there’s no room on my shelf for Isaac Asimov. I like Dickens, but that doesn’t mean Stephen King is losing my business. Diana Gabaldon is one of my favorite authors, but that doesn’t mean I’ve sworn off all others until death do us part. Well . . . maybe that’s pushing it. She’s a tough one to top.
Bottom line: there are enough buyers for everyone. If you don’t help others, there’s no guarantee of extra sales in it for you. If you do help others, then at the very least you’ll have a pretty good feeling of satisfaction that will last far beyond the next book sale.