Thank you... This was kind of my point... I will note that if you are in a situation with very heavy read usage, then replication does give you scaling regarding that. Again, my use case was for a classifieds site, where roughly have the page views are search results.
If you need scaling in MongoDB, you're looking at replica sets combined with sharding. Not all workloads, as you rightly point out, need to scale from the get-go, but there are an awful lot that need HA.