Short answers: Yes, and Not usually.
Longer answer: BCG Standard No. 21 reminds us that "the original is the most authoritative source." Are these sketchy old-school records an exception? No. Six reasons from a mainly Midwestern viewpoint:
(1) Indexers are human. They can leave something out or transcribe something wrong. This is not a rare occurrence. In this 2008 article I compared marriage indexes to each other and the original records they referred to.
(2) The licenses and returns that I've dealt with name the person who married the couple; many indexes do not. That person's identity, denomination (if any), and location may provide clues as to where the couple lived or where they created other records.
(3) They also give the dates of both events if different.
(4) Some licenses and returns give the bride's or groom's ages, or their places of residence, or both. Some also name witnesses.
(5) Sometimes the bride's or groom's ages are implied by a parent or guardian's note giving consent to the marriage. My all-time favorite in this category comes from La Salle County, Illinois (see illustration). Elizabeth Shown Mills has called such records "land mines." This one sure was.
(6) Sometimes auxiliary records such as marriage applications appear in the guise of regular marriage records; if you don't ask, you may not receive. In Indiana, many researchers know to look for marriage applications beginning in 1905, and better ones 1940-1977. Not so many know that there are two earlier forms with extensive additional information available for some counties as early as 1882.
Choose the original. You won't regret it.
Board for the Certification of Genealogists, The BCG Genealogical Standards Manual (Washington DC: BCG, 2000), 8-9.
Harold Henderson, "An Index Is a Treasure Map -- Do You Dig?," Indiana Genealogist, vol.19, no. 3 (September 2008):147-150.
Elizabeth Shown Mills, Evidence Explained (Baltimore: Genealogical Publishing Co., 2007), 16.
Harold Henderson, "Marriage Records and Indexes: Choose the Original," Midwestern Microhistory: A Genealogy Blog, posted 16 October 2012 (http://midwesternmicrohistory.blogspot.com : accessed [access date]). [Please feel free to link to the specific post if you prefer.