Q:

Would a Dyson sphere be useful as a means of power generation?

I have heard people (quite possibly on this site) say that by the time a civilization is able to make Dyson sphere it will be useless to build one for power and energy needs. So, would a civilization which can easily make a Dyson sphere make one for energy production, or would they have already have moved onto more efficient means? 
Edit: by Dyson sphere I also meant Dyson swarm, or any other thing which encapsulates the sun to harvest it's radiation energy. Also, even through the question is similar to the one suggested as a duplicate the answers I am receiving are quite different.  

A:

Your question seems to be about whether the engineering effort required to build a Dyson sphere (or swarm) might be more usefully spent building something else instead.
Unfortunately, that "something else" is not well defined. If you want to harvest power from natural sources, then your only real choices are stars and stellar remnants. The best way to extract the maximum amount of power from most of those is to wrap them up in some suitable layer of stuff. The specific kind of "stuff" in each case may be different, but the underlying principle is basically the same.
There are more energetic reactions than mere fusion, of course... antimatter is the obvious one. But antimatter doesn't naturally occur in sufficient quantity to compete with stars, so you need to harvest stellar energy to produce your antimatter. Even with the most enthusiastically theoretical physics, you're not going to get more energy out of your antimatter than you put in to creating/harvesting it in the first place.
If you open up the possibility of handwavium fringe physics, then all bets are off, of course. But with a grounding in reality, there isn't really much else you could build instead.

There are also some things that are Dyson-sphere sized, and have Dyson-sphere-sized power requirements. One of those is a Nicoll-Dyson laser: you fit the outside of each element of your Dyson swarm (or the entire outside of your sphere, if you've worked out how to make such a thing) with optical phased arrays, turning the whole shebang into a colossal laser suitable for driving fleets of lightsail starships and providing pretty much the ultimate in plausible-physics-based defense for your star system. There's not really any more plausible or sensible way to power this kind of thing than a Dyson swarm/sphere. 

A:

The Simple Answer
Yes. As far as we know, a Dyson sphere is the way to capture a star's output. Our Sun releases an incomprehensible amount of energy every second whether we capture it or not, so even if you have ultra-cheap smartphone-sized fusion reactors there is no reason to not run your civilization off of solar power.
You only maybe would want alternative sources of energy for operations outside our solar system. Not because you can't beam solar energy that far, but because the difficulty of keeping that beam focused on its target increases with distance.
A Bit More Detail
A Dyson sphere, better described as a Dyson swarm if you go by the descriptions of Freeman Dyson and Olaf Stapledon, serves two basic purposes:

Increase the percentage of stellar radiation you can capture
Increase your available real estate

The simplest possible Dyson swarm consists entirely of orbiting mirrors that concentrate sunlight onto power collectors. Every new mirror you put in orbit around the star provides more energy for building more mirrors at greater speed, so the construction of this type of swarm is generally expected to proceed exponentially. Humanity has the technology needed to start building this kind of swarm right now, but it isn't economically or politically feasible yet. Kurzgesagt has a video that focuses on this type of Dyson swarm.
Now, a Dyson swarm does not need to be limited to mere mirrors. There is physically no way to concentrate the Sun's entire output onto a single planet without vaporizing it, so to really make use of all of a star's output you need to spread out your infrastructure. A real Dyson swarm would probably contain trillions of orbital habitats and manufactories, supporting sextillions (if not septillions) of people with power and living space to spare. Science and Futurism with Issac Arthur has a video that goes into great detail about this kind of Dyson swarm, as well as its potential variations and additional uses.

