Salvete Pectinari et omnes

Beard is comparing a cultus gentialis with the state cults. That is a very narrow perception of what the Roman religion entailed. Even the public religion embraced more than the state cults alone. We hear of certain rustic festivals like the sementivae feriae, inside the City the compitales was held at neighborhood level, the Septimontium in only portions of the City, and Ovid mentions some plebeian rites that were not part of the state religion. Private religious practices embraced more than just the person's cultus gentialis. We have very limited information on what these may have entailed, snippets from a few culti gentialis where they differed from the state culti dei. Something like Horace's rite to Faunus for protecting his flock's, is that part of a cultus gentialis or a rustic rite? Either way it does not compare to a state cultus. And Horace's dedication of a tree to Diana, comparable to Romulus' dedication of the shrine to Jupiter Feretrius, but was it part of his cultus gentialis or something in addition? Roman gardens had phallic fascina and poems were written to Priapus indicating that rites were performed on his behalf. Is that part of a cultus gentialis, and if so where is the parallel in the state religion? Or is it a religious practice outside of either the state religion or a cultus gentialis?

Some things we hear about the Romans performing would be in the realm of magic, but made in the context of religious beliefs. Ovid's little rite to ward off ill rumors. Cato's healing rite by sympathetic magic. All the defixiones that have been found. Private religious practices, but not part of any state cult or any cultus gentialis. It depends, does it not, on what you regard as religious practices before you begin to make such comparisons between public and private practices?

As for the importance of the culti gentialis to Romans you might look at Cicero De Legibus 2.19-21 where he kept repeating that the culti gentialis had to be maintained. More directly, Roman law on inheritance made inheritance of the obligation to maintain a cultus gentialis a special priority. Also with the laws dealing with adoption, concern was had for whether the cultus gentialis of the adopted would be maintained, and in many cases the reason for adopting someone was to maintain your own cultus gentialis. We cannot 'know' with any certainty how important a cultus gentialis may have been to any one individual, but the indications are that for the Romans in general they were very important.

If all that is looked at is the cultus civile of the state religion and the little that is known about culti gentialis, you have only scratched the surface of the religio Romana. In between are the many dedications of public sanctuaries made by private individuals that are not part of the state or any gentile cultus. And then beyond the state cult, the quasi public cults and the culti gentialis there are other practices that would have to be regarded as part of the over all religion of Rome. I think that Beard's statement "that the vast majority of [private and family] cults essentially represented the state cult on a smaller scale," is false on two levels. First, it would be true of some family cults, certainly of some of the great families, but not I think of "the vast majority." In fact Nigidius Figulus indicated that private practices greatly differed from the state cult. Secondly, their "view of the distinguishing features of 'Roman religion'," if considering only the state cult and culti gentialis, I do not think encompasses all that can be regarded as the Roman religion. And when you include such other private practices along with the culti gentialis, then private religion did not "essentially represent the state cult" on any scale.