Harmon urges Baptist's to recognize the legitimacy of other Baptisms in other denomiantions, even paedo-baptism. He argues:
There is indeed one baptism practiced by the church in its catholicty - a baptism that includes both 'beliver's baptism' as a baptismal practice most appropriate to New Testament-like experiences of adult conversion and infant baptism as a baptismal practice most appropriate to the experience of being nurtured from infancy toward faith by family and congregation - and this one baptism belongs at the beginning of one's journey of faith rather than at multiple subsequent points along that journey. (p. 126)
He also suggests that the Baptist World Alliance should give consideration to joining the Methodists and recognizing the Joint Declaration on the Doctrine of Justification signed in 1999 between the Roman Catholic Church and the Lutheran World Federation (p.199). I think this would be a mistake since the 1999 declaration is a fudge that does not do justice to the reformed objections to the tridentine formulations. The document (which I have read in-depth) does point out some common ground and remove unhelpful caricatures, but it fails to deal with the central differences between them.
What was also interesting was that for Harmon, the main thing that prevents him from joining the Catholic church was his support for the ordination of women (p. 200).
In sum, the main strength of Harmon's book was urging Baptists to rethink the idea of liturgy and the role of tradition in relation to biblical authority. This is a book well worth reading. Other reflections on this book are offered by Sean the Baptist.