I think originally the idea probably was to have a place to engrave initials or a name -- and I've seen a number of antiques from the 1800s with engraved shields. Almost always one with a long bar shield (either with square or rounded ends).
Even so, it's quite rare even with such old knives to see the shield engraved -- and I don't think I've ever seen any other shapes of shields engraved - even from those times.
I think it was quickly realized that a shield was just the right amount of decoration for a special finishing touch. Considering now nontrivial it is to precisely fit and pin an inset shield by hand into natural handle materials, it could be considered a sign of a top quality knife.