"Inefficient" means that if you spam it, you will OOM. At my gear level, I actually gain mana by casting HW, based on casting regen and IWS procs.

For 5mans, HS is good for those quick damage spikes that you are unexpecting, and GHW is good if you KNOW a spike is coming.

Basically as your groups get better at awareness, your healing style will evolve as the need for emergency heals decreases.

What I've noticed is this:
- When someone talk about a healing spell's efficiency, they mean their mana efficiency. That means that an efficient spell is one that spends little mana per healing. If you want to compare two spells' efficiencies, you must look at its mana cost and its average heal per cast.
- When someone talk about a healing spell's throughput, they mean their HPCT (healing per cast time).

Prioritizing spells with big HPCT will increase your HPS but may make you go OOM quickly because usually high HPCT spells have low efficiency.

Another important factor to consider is the spell's TUH (time until healed) which looks at the spells cast time (and duration in case of HoTs). It does nothing to use a slow healing spell like a HoT or one that has a big cast time, even if they have high efficiency and throughput, if the target dies before it can take effect.
For Shamans, I believe (although I'm not sure) it looks like this:

- Healing Surge -> very high throughput, very low efficiency, "fast" TUH
- Healing Wave -> low throughput, very high efficiency, medium TUH
- Greater Healing Wave -> high throughput, low efficiency, "slow" TUH

I couldn't say about Riptide, Chain Heal or Healing Rain, but you get the picture...


What happens in low level dungeons is that the boss fights usually don't last long enough for you to feel the consequences of using inefficient spells, therefore you feel comfortable with only using HS.
If you want to train for endgame while you level though, try being more efficient even if you don't have to right now. Do triage, assess the situation and decide if you need to top someone off or not and avoid overhealing. Use HS only in emergencies.

I am not a healer so I could be talking garbage, so please correct me if I'm wrong. I think I get the theory at least though...