While it is true that everything has a price (and CPU calculations cost electricity, with none being available for no additional energy cost), the issue is one of scale and whether or not the results are *material - that is to say, if it is significant enough to warrant anyone to choose to use or not use ad blockers based upon it.
Looking at the claim itself, the informal study itself is here: Browser Power Consumption Quite frankly, it seems perfectly reasonable as Flash especially can really draw lots of CPU cycles (and thus power) due to the nature of how such animations work (often using vectors, numerous floating point calculations of multiple complex objects, layers, view-port culling which is not always optimal, video card tie-ins to assist in some of the processing...).
Note that every system is different, and the more energy hungry the device the more this is an issue.
But if we take the case of 11 watts, as an individual this is extremely small even if you browse 24 hours a day, all month long - then it would add up to maybe half a kilowatt hour. Given how people actually use their devices, it's probably more like 0.1 or 0.05/kwh. Here's some source information on calculating cost of energy and kwh: Cost of Electricity
However, on the other hand, this is something that an individual needs to only do maybe once a year or even less (thanks to auto-update software), and it does not even bother to even try to calculate savings in network bandwidth, or in the time savings of faster page loads and less time wasted on ads that are only rarely of any value to the individual.
Now, on the aggregate this is a bit more compelling, and could be considered a material factor to consider for IT departments on large campuses, government agencies, software vendors (like Mozilla including the plugin by default...), etc. On a campus of 10,000 people with thousands of workstations, a 0.05/kwh/user/month savings could shave 500/kwh off an electric bill, save lots of download bandwidth, etc.
Back on the other hand, note that is all only considering a desktop PC, which is the most power hungry internet-consuming device in existence today. The iPad 2, by comparison, has a battery rated at a maximum capacity of 25-watt-hours source, and makes an end run on all this by simply not supporting Flash in the first place - though it still supports AJAX and other types of animations, of course, so is still somewhat served by ad blockers.
The authors themselves say there are far more potent ways of saving energy, but it seems to validly be another small - but measurable and real - benefit of ad blockers.