Arithmetic checksums are weak - there is no argument about that.
I was just pointing out that the MessageImprint type could be broadened easily to accommodate the various needs of clients of a timestamping service. The first mode is obvious. The second mode allows time stamp tokens to serve as standalone data vehicles that can be used to verify the content as well as carry the content - this would typically be useful for small messages. I do not think that arithmetic checksums are useful. It is trivial to generate two documents that have the same arithmetic checksum.