I have a number of marriages that I identified through the civil registration indexes in the London area. Since there are a number of churches in each district it would be unlikely that I would find the specific church the marriage took place in. I don't have access to all the church records nor the time to search them. I was going to have to buy the certificates from the registrar office.
How complete is this ancestry marriage index? I found 12 marriage records of the Scoggins that I am researching. I found almost every marriage I was looking for. This tells me that this index, linked to images, is very extensive. Yes, I was able to download the images as well. I also found a marriage not recorded in the civil registration indexes. My pioneer ancestor, William Bye Scogings, married Sarah Raper about 1850 in London. I found that marriage in the parish of St Mary, Rotherhithe. Now I have the certificate. I thought I would never find that record. He joined the Mormons in 1853. Sarah never did join. The story goes that he had to choose between his family and the Mormons. William left his family in 1859 for Salt Lake City never to see his family again.
It use to be that a methodical search was required to find most records. Now with the indexing that is going on it is increasingly only necessary to know the name and the place and the search engine finds the records. The new term added to the genealogist vocabulary is 'wild card'. The methodology of genealogical research is changing with this massive indexing effort. It pays to know the subscription sites. Library's now have subscriptions to some of these sites so it may not be necessary to buy one. The largest free site is familysearch.org and there are many others as well.
I would imgine that a small portion of the records in the world are indexed, and the subscriptions sites focus on those records of most use to the researcher ranging from census record, church records, directories, military records, etc.