So, when determining the value of a card in Hearthstone, you have to start by looking at the value of the card's abilities. Let's look at Harrison Jones' abilities seperately, and determine whether the card is worth it or not.
So let's start off with it's base stats. A 5/4, with no other abilities would be considered pretty decent at about 4 mana. This assumption is made due to the closest two minions in stats with a lack of or neutral beneficialness abilities are the Chillwind Yeti, at a 4/5 for 4, and the Ancient Brewmaster, a 5/4 with a sometimes beneficial, sometimes negative effect.
Next, let's look at it's abilities. This part is typically difficult to value concerning legendaries, since their abilities are often quite unique. Harrison Jones is a bit easier than most, due to his abilities being very straightforward as well as being on a couple other cards in a way, but still requiring speculation. The ability to destory an opponent's weapon on battlecry is a very powerful ability at times, but is definitely a situational ability. Acidic Swamp Ooze, the other card with this part of the ability is a 3/2 for 2, a fairly decently costed creature on it's own. This implies that the ability to destroy an opponent's weapon is valued low, perhaps not even enough to take into account for it's value. However, for now, let's simply say it's worth a half a mana, because of how game changing it can be from time to time.
It's final ability, the card draw for destroying an opponent's weapon is very situational, primarily due to the fact that most weapons tend to have 2 charges on them, so more often than not, when you destroy an opponent's weapon, it will only have 1 charge left, netting you only a single card. However, as seen on cards like Novice Engineer and Gnomish Inventor, 'Battlecry: Draw a Card' is worth about 1 mana when stapled to a creature. Again though, Harrison Jones only on occasion draws you a card, but at least it has a chance of drawing you multiple cards. So we'll give it the benefit of the doubt, considering how strong weapons tend to be, and put this part of the ability at 1/2 mana as well.
So let's look at what we've got for Harrison Jones. We have:
- 5/4 - 4 mana
- Battlecry: Destroy opponent's weapon - 1/2 mana
- Conditional 'Battlecry: Draw a card' - 1/2 mana
- Total: A bit generous of 5 mana
So While Harrison Jones isn't necessarily one of the most 'efficient' minions, he is definitely a viable option for decks for the same reason the Acidic Swamp Ooze is, for those edge cases where his Battlecry ability saves your behind. The difference in mana cost isn't a huge deal, considering most troublesome weapons cost 3 or more anyways. So if you have a spot in a deck with a generally low mana curve, I would definitely consider running him in the deck.