While a Necklace of Fireballs does come with some downsides - cost and fragility - when compared to a wand or scroll (though, of course, scrolls have their own fragility issues) of Fireball, the Necklace does carry with it the advantage of ease of use.
To use a Necklace of Fireballs, one does not need to make a Use Magic Device check, nor does one have to fumble around in their pack, scroll case, sleeve, or pocket as one might typically need to in order to retrieve a scroll or wand. The Necklace also does not occupy the throat slot while it is being worn.
This advantage of accessibility (and fragility!) might seem a bit underwhelming at first, but consider a crafting wizard with the feat Leadership - such a wizard could equip any of his followers with a Necklace of Fireballs and be confident that the Necklace will get used. (And, as a crafter, he would be saving 50 percent of his cost for equipping his followers.)
Or, consider a DM handing out a loot pile - many DMs (myself included) like to roll for loot piles ahead as part of preparing an adventure. When a Necklace of Fireballs shows up in the loot, I can be confident that that item will get used by someone, as it requires no special skills or training. That ease of use makes awarding loot more streamlined.