With Hobies, the sail numbers are more like serial numbers of the sails themselves and are independent of the rest of the boat. I've ordered replacement sails for both my 16 and 17 with the same (old) sail number - and it cost extra (both money and time). Most people aren't that attached to sail numbers.

The masts have serial numbers as well - independent of the boat.

It's not so much of a marketing tool as it is a tracking tool for warranty and the like.

With the exception of the very early boats, you can always tell the age of a Hobie by the hull serial number.