First, comparing it to Hungering Arrow. Hungering Arrow's EV might be around 250% damage, whereas rapid fire is only 220%, but so far Hungering Arrow is far more wasteful. That 250 dps is assuming it does full damage to every target it goes through and that isn't the case. Most monsters only have half the health of a HA shot so you're wasting half of its dps every time it pierces and hits someone new. Rapid fire concentrates on a saturation of quick fire at low damage. This way each arrow hits for maybe 3/4 of a monster's health and there is less wasted damage when it kills a target. I find I clear rooms much faster with rapid fire than I do with hungering arrow.
I also find it more useful than HA when trying to pick off a single target in a group. Using HA will not burn one guy down because it will pierce into everyone else, effectively losing dps when you are supposed to be concentrating on burning down that specific monster (aka a summoner).
I know Hungering Arrow is a generator and this is a spender, so let's compare it to Impale. Comparing it to Impale, is more or less a comparision of its overall utility. On a single mob, like the SK, it is always going to be worse in absolute terms because its weapon damage is much lower. But if you are put in a room full of guys you're going to have the same problem with impale that you do with hungering arrow: overkill. You don't need that much damage. Also there is only an initial cost of 5 (runed) for rapid fire along with a sustained cost of 10. Impale costs 25.
Rapid fire also helps circumvent the problem i mentioned two paragraphs above, similar to the one encountered by using HA. If you're trying to burn down a single target in a mob and you can hit him, then impale is the way to go. What I often find happening, however, is that the target I'm trying to focus (say a summoner) summons someone right in front of him - or someone just steps in front of him - and impale hits them instead. To me thats a waste of 25 hatred and a missed opportunity. Rapid fire allows you to waste minimal hatred to quickly kill the monster in your way while moving on to the target you really want to kill. Because it is always firing, you will at least get some dps off before something else gets in your way. This problem will be circumvented when impale gets the pierce rune, but for now RF is the way to go.
Because of this, I find RF to be a very useful tool. There is always going to be a spell that dominates numbers wise (aka impale) but I feel rapid fire is still a viable alternative and actually performs better in certain situations. Not to mention it is just insanely fun to use.