Adblock isn't an "expensive extension". Ads are the expensive thing, so blocking them saves power.
I believe Firefox partly resolved the problem on their end later on, although I can't tell you the exact status of things. Nevertheless, there's definitely some precedence for claiming that ad blocking is an expensive operation. Intuitively, you'd think that network based blocking would be enough, but it won't work against same domain ad sources (a typical example being facebook ads), while css selectors are able to capture a bit more depth. Nevertheless, I don't know exactly what has happened since, although I do recall Firefox making some adjustments on their end that improved the situation.
[0] http://www.extremetech.com/computing/182428-ironic-iframes-a...
[1] https://blog.mozilla.org/nnethercote/2014/05/14/adblock-plus...
The only reason I can think of for someone not using uBlock Origin is because they've never heard of it.
I've only ever had to whitelist two things in uBlock in all my time of using it and that's pretty good considering that I use basically all of the lists that aren't the language specific ones, the anti-anti-adblock one (which requires a user script) or the merged (ultimate) lists.
But yes, my browsing habits are clearly different from yours and what works for me may not for you. I understand that but what uBlock does is not any different than any of the other adblocks that are out there. It just happens to do it more efficiently and with a better UI than the rest of them. The only difference in behavior that you might encounter is likely to be related to uBlock Origin's strict blocking[1]. In this case exception filters are very easy to create since you literally just have to press the disable permanently and it will forever be disabled for that site.
I've said this before the the best way to use uBlock Origin if you've never used something like NoScript/uMatrix before (or couldn't be bothered with the whitelisting approach) is to try and use what it calls "medium mode"[2]. Using its dynamic filtering in this way should net you with the largest gain with the least amount of effort. If you're looking for something with more control than you may want to look into uMatrix since I think the interface will is nicer for that sort of control.
[1] https://github.com/gorhill/uBlock/wiki/Strict-blocking
[2] https://github.com/gorhill/uBlock/wiki/Blocking-mode:-medium...
The power button only disables it for one site at a time (or even just a single page, if you Ctrl-click it).
[1] https://github.com/gorhill/uBlock/wiki/Blocking-mode:-medium...