Ancestry has recently added two new databases useful to those with a local connection. The first is the Missouri State Census Collection, 1844-1881. These census records are a helpful way to fill in the gaps of the federal census records. Prior to this release, the records were not widely available though the indexes were. Actually, microfilm of at least some of these records is at two or three places in the St. Louis area.
But there is more. While it's true that many of the state census records have not survived the years, the Ancestry collection is not complete. For example, I believe census records for St. Charles County (1852, 1868 and 1876) have been indexed within the last couple of decades, suggesting that the originals are somewhere waiting to be filmed or digitized.
The other database is Missouri Birth Records, 1805-1980. I've used the index and microfilm to the St. Louis records in the past and had very little success with my families. As Ancestry notes, only about 60% of the St. Louis births were recorded between 1870 and 1910.
The inclusion of births past 1910 must be the result of miscellaneous records happening to have been filmed. I can't see the number of those records growing, what with those records not being open in Missouri. The database title is accurate, but don't get excited hoping to find a treasure trove of post-1910 birth records.