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When using PPPoE the 255.255.255.255 mask is correct.
The purpose of a subnet mask is to define a network address, host address, and broadcast address. With a mask of 255.255.255.255 (also called a "host route" as defined by RFC 2328 found here:
RFC 2328 - OSPF Version 2 (RFC2328)) the IP address becomes the network, host, and broadcast address effectively passing whatever traffic it receives to the next host. Based on that information, your ISP is correct by saying that this configuration is necessary for routing purposes.