The current Guild Task System is broken. The have to change that you get the SP for the class you are at the moment (when you give in the items). You could do a rank 50 task with a rank 1 w/e and get tons of SP. That's why I think that there are currently only low level tasks and sp bonuses.

You are right about the fact that they are not really task, because you can buy the items for a few gil (NPC/Bazaar). So Miner/Botanist/Fisher tasks should be somehow different than the DoH ones, where you actually have to craft the items (although you can buy some of these items aswell).

Actually I would like to see some quests for the ranks ~5/~15/~25/ because most leaves range ~10/~20.