If you're already very energy-conscious, I can see your point. However, most people probably don't think about how much power their standby (and operational, but unnecessary) devices consume.
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.
Well, I don't think those people would even buy such a device. Only people that are aware of their energy usage and actively want to reduce it would want a device like this.
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.
There are several reasons you might want live power monitoring:
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.
In the typical case such a device is really only useful in the first month or two. After that all optimizations have been done. So I think that this monthly subscription model is a bit flawed in that regard. I'm probably wrong..
While I too have doubts about WattVision's monthly subscription model, I can assure you that an energy monitor has surprised most of the people I know who also have them -- they become ingrained into your life.
Power monitoring can still be useful to help prevent you from slipping back to inefficient usage.
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.
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.