I don't see much other use for a device like this if you've already eliminated standby devices. Sure, it can tell you that your fridge is wasting alot of electricity, but that's what the energy labels are for (A++ etc)
An analogy: I don't need Mint to tell me I waste money. Of course, I try to minimize the amount of money that I waste, but some is inevitably spend on stupid things. However, Mint is a tool that helps me identify exactly when and where I'm wasting the most money, which informs the financial decisions I make.
Of course if they got a deal with an energy supplier and got this installed for free in all houses it'd be a different matter entirely.
If you insulate houses for a living and want to prove that the heater is switching on less often (paying off the cost of the insulation) you want live power monitoring on that one circuit. You can then correlate the power saving with outside temperature and time of day.
If you run office buildings, real time measurement can help you localise faulty HVAC units. It can help you find out when the lights are on and they shouldn't be. It can tell you that there are a lot of standby loads being left on overnight.
If you have a typical house and a huge power bill, you might not know which of many complex loads is using the most power. Perhaps it is standby devices. Perhaps it is lighting in the kitchen. Perhaps the fridge is leaking and turning on more often than it should. Maybe it is the hot water. Once you have localised the problem, you can make a smart investment to save power.
There are a lot of measures that can reduce people's power usage for a short period of time. For instance, advertisements to ask people to turn off unused lights can have a temporary effect on power usage. Making those savings last is much more difficult. Hopefully ongoing power monitoring can help there.
For Gridspy, I'd love to provide a free minimal service, a highly affordable normal service and a premium service with long range high resolution data.
I assumed there was no way I could use less electricity. I was completely wrong. Think of it more like a thermometer or a clock.