The first question would be: how do you know yours is a machine-made blade?
Second: do you know when and where it was made?
Also, the gentleman you were talking to is incorrect; many machine-made swords had signatures and other inscriptions. The variety was quite wide. Samples of a half-dozen oshigata (tang rubbings) from various shingunto and gendaito are shown in the back of Fuller's & Gregory's Military Swords of Japan: 1868--1945 for example.
As to "why" -- even if a blade is machine-made, there is a great amount of hand-fitting, polishing, etc. that goes into it, and signing it could be similar to machinists at Ford signing a car body -- not just identification, but a mark of pride in workmanship.
HTH.
Last edited by Brian Owens; 1st March 2006 at 11:15.
Yours in Budo,
---Brian---