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Actually, there was one place where the Mage "slow fall" spell was something I used consistently. The Twin Peaks Capture the Flag battleground (I hope I remember the name correctly). There was a fairly high plateau there and if you had slow fall, it became a quickish escape for you and fall damage for anyone trying to follow you. I don't consider that implementation of reagents very good.
Now, I will freely admit I wasn't good at the game, I didn't play that mode for long and this is already fairly niche. Nevertheless, I am of the opinion that if you make a reagent using spell something neat and flavorful for most players and integral to players of the same class but doing DIFFERENT content, you need to make this a part of a healthy economy for your game. Otherwise it's frustrating. In other words, it should allow some players to think to themselves "the mages are probably going to care about this reagent this month for whatever reason, so I should go gather it to make money off the Auction House". The mages should then be able to decide for themselves whether that's something they want to spend the gold on, or gather for themselves ahead of time.
And even then, if you asked me to design this kind of thing, I wouldn't do it for the slow fall type stuff of all things. It means you would have to carry those reagents around for possibly months at a time until the niche situation arises. Most players will sensibly choose to make better use of the limited inventory slots and then be frustrated when the situation occurs to do something cool as the class they play, but they chose not to carry the reagent. And then you also have players who literally use it all the time because they do very specific content. The better approach imo is to make it required for something somewhat rare, but more uniform across classes and/or content. Like say... Leapfrog Potion to jump over ledges when you know a rare but notable part of exploration involves taking shortcuts over ledges. That suddenly becomes a neat cool thing for most people to do sometimes and an actual job requirement for someone making profit off exploring/gathering (assuming the game allows for this and has a good economy).
Edit: I suppose for slow fall type stuff in a game like WoW where it barely ever comes into play for most people most of the time, another sensible approach might be to introduce a second use for the same reagent, to make it something people can expect to use and need to carry around more consistently. Then it once again becomes a question of stack sizes, how well the game economy is designed and whether you feel it makes the game more fun.