I like to say that this is only my opinion, and that even the image, is all party of my reflection and interaction with the game, being a cleric class.
Personally for me, save exp cards is a waste of time for some classes, like cleric class.
All you do by saving cards is replacing the time you will be with difficult to leveling, for grind much before, what may give you the perception of that you are going more fast, but is just a replaciment.
Also for some classes, you will waste 5-10 seconds to kill a mob, so later you need only 1 seconds. When you count all the monster you lose this time, you see that you also lose time here.
Anoter example: doing all the quests, following the Kaipleda route alone, you can reach LV 130 more or less close to Femidian. People could go back and do the Osha route, but will lose too much time doing that. Or they can continue. But in fact, since they are already on a highter level than “Orsha leveling route”, no matter when they do it, they will lose the same ammount of time and get the same cards.
The same way, while you keep the cards for the future and have more difficults to kill mobs, in the future this cards will be only a small percentage, almost nothing, needing hundreds to make any difference, that mean if you already had used this cards, you my had reached the same level much time before, and the only problem will do when exp curve comes, you will need be more patience, but is not that you will be stoped in time or lose more time, is just that you dont replaced this time grinding before, having dificults to kill mobs.
Completing maps and monster’s kill missions in the adventure jornal is even more easy in highter levels, since will need just 1 hit for most part of the mobs, the by creating recipes you will be already completing this missions of the adventure jornal.
Using cards: you will lose the bonus of griding on the same level as mobs. BUT in compensation, you will end the quests and take the cards much more fast if you do it in a highter level. So again, is “Time lost vs benefits”
The only way I see is worth save cards if you have difficult to grind is:
1 - When you be close to get class level 15, because if the card give you 7 percent and is only 3 percent left, you may want to grid this 3% left instead use the cards.
2- In dungeons, is better to go at least 6 times on the dungeon of your level before continue to use the cards, because dungeons do a much more exp than grinding (for classes that have difficult to grind), so plus the bonus of exp, this worth it, but is only a few times, I think 6 runnings into the dungeon is enough to receive lots of levels and continue the journing.