SP obviously chose not to have fields for the parents' names for the statutory indexes
I don't think that this is something that SP 'chose'. It was imposed one them by the way the indexes were made up when statutory registration began.
In the
good bad old days, when you went to New Register House, all the indexes to the statutory registers were heavy tomes, the earliest ones handwritten, that you hauled off a shelf, consulted and then heaved back on to the shelf again. Until 1929, mothers' surnames are not included in these tomes.
Some time later, these entire volumes were digitised, that is, the data in them was computerised.
Later again, the internet came along and SP was set up using that index. It would be a major task to go back and re-index everything from scratch, and I'm sure the indexes as they are now are better than no indexes at all while they find the resources to improve them.
I see that mothers' maiden names are now gradually being added, but this is going to be a long, slow and expensive process as it requires indexers to go back to the original books. It has been suggested that the GROS could get volunteers to do this, and that it would make more sense to start at 1929 and work back rather than at 1855 and work forward, but these things take time.
Meantime, in the late 1970s, the LDS were allowed to create, for their own purposes, an index of records over 100 years old, as part of the IGI. The purpose of this index is to 'seal' children to their parents and spouses to one another as part of the LDS' own procedures, but the LDS have been kind enough to make this index freely available. It covers only the first 20 years of the statutory records of births and marriages, plus two later years, so although it is a fantasic resource, it is limited in its time span.
Now, the OPRs. The LDS also indexed these, but it's a bit complicated, and I may have got the wrong end of some sticks. For a start, although northern counties are, in theory, fully indexed, there are some curious gaps. For instance, the IGI completely omits the entire register of baptisms in the parish of Duffus from 1820 to 1854 - it looks as if they just forgot to do it - and there may be other similar gaps. Then the southern counties were not completed as part of the project to index all of the OPRs by the time the IGI extraction programme ended, so although many of the southern counties have been indexed, they are often in the 'contributed' section rather than the 'indexed'. In some parishes, the LDS appear to have indexed all the baptisms of males but not females, and in others it's vice versa.
I think that the SP OPR indexes were based on the LDS indexing, but with additions, because the southern counties appear to be complete (unless someone has evidence to the contrary). My reason for thinking that the LDS OPR index is part of the SP OPR index is that the Duffus baptisms between 1820 and 1854 were (and possibly still are) missing from the SP OPR index too.
I may of course be quite wrong about some or all of this. I write only as I have found using these various resources over the last 30+ years.
See also
http://www.rootschat.com/forum/index.php?topic=714261.0