I agree it is not worth spending 5000 sapphires for a single goldsmiths use.
I consider the improvement too limited and many players I had asked about the best use, prefer to ignore them instead. I prefer, for now to use only crystals level 4, so I can exchange them any time.
All unit type has a moment where it is more useful and so having an "specialty" on a single type is hard to consider useful, where it is better to make more troops instead. Maybe for the highest levels that strict and limited improvement might be useful. Maybe highest players are totally focused on maximizing their beasts attack or defense, in that case I think that beast defense would be the best crystal so maintaining alive more dragons and griffins. It would be great if it is clarified if the defense crystal will help in both attacks and reinforcements to reduce the number of units lost.
Before that moment, I prefer to change crystals many times, keeping them at level 4.
There is nothing in the game, at least so far, in terms of game balance, that would make some people to prefer one crystal over another for the long run and then have a sudden reason to change it spending real money. Maybe someone thinks that this is warranty that players will consider spending twice, not just once, so they will change again to their original preference. But then, I see nothing in the game that will encourage a player with a high level active crystal to exchange it for a temporary event and happily pay for that.
Finally for making this a constructive post, and not just blaming and criticizing, I think it would be better if:
a) Goldsmith could be like Artisans and we could pay for them for infinite uses. Maybe there should be a better goldsmith every 3 levels so you have a profit.
b) Add some patterns or situations where players can acquire the mentioned wisdom where it is an advantage to spend sapphires on changing crystals. Maybe improve the percentages or add more benefits. A crystal might still be focused on ocultism attack but can give lesser bonuses on ocultism defense, and infantry and cavalry attack.
c) Add more variety to crystals and make them less tight. Have only a crystal for each unit type, affecting both attack and defense. Or make more crystals, maybe some common and some rare to find crystals. A rare crystal could give attack bonuses to all attacking units, or even better to give defense bonuses to all attacking units (less attacking units will die, it depends on the complexity of the combat algorithms...). Some crystals could give bonuses at battlegrounds, others at settlements, others at beacons. Some epic rare crystals could give league bonuses so percentages should be really small, so as more league members participates in an attack or reinforcement the global improvement for all members will be higher, encouraging teamwork.
Well, I hope this could be helpful and creators, designers and programmers consider them useful.
Despite the disagreement on how goldsmith's functionality currently works, I am happy with most other game features. Thanks for a great MMORPG and for your rising motivation to keep improving the game.