It's a fair bit easier to setup than Cassandra and a document store is simply easier for most people to reason about than column-families.

For something like your typical rails app, Mongo has some real nice properties. It makes it sticky. You start out with Mongo because you can just drop data in and off you go. You keep using it because Mongo, despite all of its drawbacks, is really plenty good enough for more than 80% of the stuff on the web.