Other reviewers have criticized the authors for it, stating that it narrows the field of research too much. Moreover it will naturally preclude the option of Arthur being found anywhere but in the bards' own locality.
But to my mind, it's an approach which makes sense on so many different levels.
No-one ever pretended that Arthur was anything but Celtic, however much he was dressed in Medieval French armor or placed into an Anglo-Norman castle. Everyone has always bemoaned the fact that much was lost due to the Celts having an oral tradition, without mentioning that it was the Bards who passed on those stories.
Preserving history and legends is what the Bardic system is all about! They prided themselves on memorizing their heritage in oral form, then passing them wholesale to those who followed. This is why there were strict bardic meters, and a tendency to group things into Triads. They served as mnemonics, and made it immediately obvious if something was missing or had been added in.
If any class of people had undiluted access to the facts about Arthur, then it was the Bards.
Mentions of Arthur are plentiful in the evidence left by the Bards in Wales. Some of whom witnessed the scenes that they were describing, or else wrote in the centuries immediately afterwards, based upon legends passed down in a Bardic meter.
Ignoring them has been like telling the story of Crazy Horse just from General Custer's point of view, while treating Lakota perspectives as irrelevant. Or only permitting Taliban sources to evaluate the events of 9/11.
Moreover, it's well established that English and Anglo-Norman writers have historically twisted Celtic legends - particularly those involving Arthur - to serve their own political needs, or else negate the threat in inspiring the Celts to rebellion. Starting with Geoffrey of Monmouth, who placed Arthur's birth at Tintagel, home of his Norman patron's brother; and including the most famous version, penned by Thomas Malory, who used the chivalrous Utopia of Camelot to obliquely criticize Plantagenet monarchs embroiled in the War of the Roses.
Their efforts rendered it acceptable - indeed obligatory - to always update the Arthurian legend, without regard to historical accuracy and/or a Celtic context, in order to express an alternative narrative.
This tradition continues into the modern day. George Lucas used it as inspiration for his Star Wars movies, which some have viewed as an ideological commentary about Capitalism versus Communism; while a Feminist context is blatantly apparent in Marion Zimmer Bradley's (admittedly fabulous) best-selling Arthurian novel The Mists of Avalon.