It is often very valuable to do so, no denying it. In fact it might USUALLY be the best call. HP is a resource that can be spent freely as long as you have enough of it. But in the end, you spent 4 HP, 1 mana, and half a card to remove 1 card that had already dealt 4 damage. The opponent spent 3 more mana and half a card, but dealt 8 damage to you total with it. Against a beatdown deck, it can often be better to send in a minion or use a damaging spell and go 1-for-1 and allow your opponent that 4 damage, because generally you will get your value back in spades when you hit your superior lategame (assuming you aren't also a rush deck)... but this won't work if you die before you reach that point.
And being at 'low HP' is often arbitrary when playing against combo or rush decks, and you often you should strive to just save as much HP as you can. As noted above, some people just have ridiculous combos. There's a notorious, high-level deck known as Miracle Rogue that can consistently deal 26+ damage in a single turn around turn 9 or 10, and the entire deck is built around drawing into the combo while simultaneously removing all of your threats.
And being at 'low HP' is often arbitrary when playing against combo or rush decks, and you often you should strive to just save as much HP as you can. As noted above, some people just have ridiculous combos. There's a notorious, high-level deck known as Miracle Rogue that can consistently deal 26+ damage in a single turn around turn 9 or 10, and the entire deck is built around drawing into the combo while simultaneously removing all of your threats.