MongoLab is actually a DBaaS (DataBase as a Service) not a PaaS, just for clarification.
The reasons for hosting a database etc offsite is similar to hosting files offsite with say Amazon S3. You are looking for a service that specializes in what you are using it for. MongoLab specializes in MongoDB, sharding, replication, large data sets etc. They would be a great provider if you need those services. If not, then the MongoDB instance on OpenShift should be fine, you can even use a scaled application to get into it's own gear, but we do not support sharding or replication for MongoDB at this time.