Good points raised here. Why would one want to increase the speed of levelling? Why would Nevrax give us the possiblity of leveling faster? - faster as in noticeably faster; of course, other factors like proper gear, knowledge of the xp and mobs system etc. are an advantage and do have a miiinor impact as well - (just my thoughts, subjective and biased, of course) to me it's simple: when levels themselves become players
goals and not a consequence of players actions or when levels are
requirements for players goals, and we are impatient to fulfill these goals. For example, lack of other things, a player may define his goal as crafting q250 armor, or a PvP player may see being high level mage as a requirement to take part in faction PvP, and it simply "takes too long"
Like it or not, global PvP, outposts and spires are one of the most cost-efficient ways to extend Ryzom given low budgets, reduced number of developers and financial problems. Other than the Ring, still under development, we have mostly seen PvP oriented new game features - despite the "Some of you don't like PvP at all. Period. End of Story (snip) we'll see more PvE, kitin behavior (snip) " letter we had months ago -. It's somehow Nevrax bet. Extending Ryzom in other ways and in a timely manner to keep a fraction of the playerbase busy and happy requires a lot of resources.
So, with PvP content (ultimately unlimited and reproduceable) the speed of leveling becomes important -- one may say that in a pure PvE environment, with a large population, and without other players to "compare to" speed of leveling may not be a concern? -- since a fraction of players liking PvP may not be willing to level up taking the time that it used to require. During early stages, we were said that the planned time to get a lvl 250 was 6 months of regular play - 20 h /week, iirc- allowing players to take part in this "high level pvp only content" requires players leveling faster. Crafting and Harvesting, unfortunately activities reduced to backup fighting skills, need to level faster too, directly and strongly tied to fighting.
Crafting: it seems to me that we were supposed to be "Master Boot/Gloves/... makers" not "Light Armorers", players would specialize in a piece of a set, rather than a whole set (just think of this, on one hand, and before the jewels were revamped, if a crafter exclusively crafts 'sets' his levels will diverge, on the other hand, truly high level crafters able to craft high quality sets without overcrafting are hardcore/dedicated players, some after even 1 year of crafting regularly). The use of crystals reduces the insane amount of hours and mats required to equip the high level fighters, and it is required when leveling faster.
Harvesting: for either a) economy b) craft-feeder or c) exe and sup hunting. One would want to increase level to get higher quality mats faster, dig required supremes and overcraft at an optimum delta, also tied to the aforementioned points.
So I agree with previous posters, with some exceptions, crystals are proposed by devs to reduce the time it requires to hit cap, and the risk is new players not enjoying the trip.
Personally, I usd them when I can get them, since I have enjoyed the trip, and as crafter, once the recipees have been found, the lands have been mapped and filled with flags and it's just the grind to meet a demand, it becomes boring.
Edit: Soooo,, with all this dissertation

I really don't know what to vote. I use them and enjoy them specially when the gameplay becomes less enjoyable. But if I had to say something... I'm aware that op rewards need to be worth it, but regarding crystals, I'd go for something like Ep2. Give crystals as a reward for something that took time to do and, to some extent, compensate for that, allowing more flexibility: if I hate digging but I love crafting, doing something more enjoyable for me like killing beasties in an invasion, although I mastered that branch and then trading honor points for mats and crystals is a good example of how I'd like them to be used, and in a macroscopic view, with the same impact and rate of leveling I would have had if I had spent the time digging. The craft levels I got when using crystals bought with honor points I could have got them digging craftable mats rather than temple ones.