I thought about this before when I was engraving my R8 ring for better stats. As I kept thinking about what would happen next if this was implemented.
1. Game Economy would be destroyed. Everyone would be able to engrave ornaments as much as they want and this would literally prevent people from selling good engraved stuff.
2. Nobody would join Nation Wars. One of the reasons why people join NW is because of it's ST rewards, do you think everyone would join NW if ST can be bought so easily?
3. Empty GM events. People usually join these events for fun and the prizes given out. People would just convert all their ST to Event Tokens and events hosted by GMs would be empty.
ST is one of the items that keeps people bonded to the game. Since we have people going for max engraved stats, people would just donate and engrave till they get their wanted stats.