"I would like to welcome to this blog our newest contributor" is something that you will never see at On My Mind so long as this is "my house." While I may consider entertaining a guest contribution from time to time, regular posts to this blog will always be made only by me.
Bloggers know that in order to increase viewer count, one needs to make rather frequent posts, and that tends to be pretty hard work. One way of avoiding the hard work of coming up with topics and putting together reasonably intelligent pieces of discourse on those subjects is to spread that work out over as many people as possible. The problem is that the space becomes no longer what a blog was designed to be; a means for one person to communicate with the wider world.
My experience is that the added contributors do not reflect the character that led me to the blog to begin with; be that a sense of humor, a specific point of view or, more usually, a specific writing style. Thus, as more and more contributors are added, the less I am attracted to the blog. I find myself skimming for posts contributed by the original author, but even then all too often the style and flavor of writing has been corrupted by that of the added contributors.
There are very few multiple author blogs that I continue to read.