I agree, use them if you have them! I have a major collection of Plomb WF series tools that don't get used. I'm piecing together this collection as a museum piece and want them in the condition I find them. All my old BE Craftsman, Bonaloy, Bonney Streamline, Vintage Wright (-42 and earlier) and Plomb pebble era tools get used as needed.
I don't think Plomb ever came in metric sizes. I know there were some Whitworth sizes producted. Not sure they were in Pebble though. If you want metric, I think you will have to go with the post Plomb Proto versions. I'm not sure what year they started producing them in metric.