Why do Agricola and Meurig disagree with Arthur's plan to fight at Lugg Vale in the book, The Winter King: A Novel of Arthur?

Agricola disagrees with Arthur's plan to fight at Lugg Vale because he sees it as suicidal. Meurig, on the other hand, pedantically asks why fight Gorfyddyd at all, since he has no designs on the Dumnonian throne.