I sometimes get the key items tab at least mostly full when I pick up a dozen Ixal quests at a time. They use between 1 and 3 key items each, and I'll usually do the gathering portion for 6 quests at a time, and then do the crafting portion (which uses up the key items and gives inventory items in their place). Adding up two or three key items for each of half a dozen of those quests, along with a miscellaneous item or two for whatever else I have going on at the time, can take a big chunk out of our 25 key item slots. Someone could certainly run out of key item slots if they had a wider variety of things underway at a time than I generally do.