Araye, I cant really say why, but:
alchey is not exactly the ame as magic, it is using it in some other form, and also in some more real way than magic, I'd say alchemy is half magic, half chemy of our real world. Also because its like storing magic powers into a vial of liquid substance, it may be axpected to be harder than a pure magic, cast directly by a mage. Maybe as it takes time to store the magic power into the potion, it also takes time for it to take effect. While a mage may control how he heal, the alchemist can't really control it while he make universal potion for many and very different kinds of wounds.
The better the alchemist, the better potions he does. More HP may be restored wit one potion and its effect may be also shorter. Bad alchemist will make it good, but its effect will fully end after 30 minutes, while it will maybe resore many HP at that time, master alchemist will make a potion restoring the same amount of HP, but it will take 5 minutes max.
I know it is preferred way in every good RPG i have pleasure to play.
Ultimately i prefer this concept, coz my char is alchemist and I have been paying attention to different ideas about alchemy since some longer while. Basing on this I can say that complex ideas, with many different factors, like the one above, gives more realism and in consequence more rp opportunities. We can play with our chars a role and be really experts about it. Not because of stats and skills, but because we really know what we are saying and it won't be obvious for other, coz the job your char is doing isn't that easy to master just like that. Also a way to encourage specialisation.