It is not necessarily better. I have mine powered by a USB port. TV goes off and so does the dongle. That means every time I turn the TV on it reboots. No real issues for me. Goes back to the age old question: Should I turn my PC off when I am done or leave it running. Some people swear you should just leave it on. I have been turning mine off for years and my last one lasted over 10 1/2 years. I only replaced it because it was old. Still ran fine.
The PC comparison isn't accurate at all. You don't turn off your computer by pulling the plug on it while it is booted up and running. You wouldn't be using that computer for 10 years if you did. And if you did that while you while updating the firmware on your computer, you wouldn't be using that computer for 10 more seconds without a great deal of work and expense to fix it.
The most important factor that makes using the AC adapter preferable to using a TV USB port is that if it shuts off with the TV you could inadvertantly be shutting off power to it while the firmware is being updated. So if you are going to use the USB port, you should make sure right before shutting off your TV everytime that it is not in this process.
Now there are some TVs that power the USB port even when off but if the OP has one of those it won't help them then for their purpose in this thread at least. It would be best for them to just plug in the chromecast only when being used and then unplug it after if they need to be concerned about usage limits. Most TVs right now do shut off USB power when the TV is turned off but everyone should check for themselves on their own TV if they are unsure.
So, other than that main reason listed above, there are also other reasons that will not affect everyone, depending on your TV and the rest of your setup. Some people experience interference when plugging it into the TV. Some people have had various issues such as no audio or problems selecting inputs when USB powered. Every TV is different and has different bugs and also can be purposely designed with power saving features such as shutting off the usb port when it does not think it is in use (typically according to hard drive standards, not media players), etc. Ideally, you don't experience any of these problems, but some people have found their TV does not work well at powering a streaming device through the USB or that the proximity of the USB and HDMI port causes problems.
There is also the potentially longer wait for updates. It can not update when the power is off and if you are using it while it is on, it does not have a chance to update. There's also the inability to use CEC. Not everyone has CEC on their TV and not everyone wants to use it (some even have bugs with CEC that make it better to not use it on their TV), but if you do want to use it, you have to plug it into AC. I use CEC not only to switch inputs when I start casting, but I also use it to have the chromecast turn on my TV every morning while I am still in bed in a different room so I can use my wireless remote app on my phone to turn on the news. It comes in very handy for me personally.
Anyway, the most important reason, if you are not experience any bugs related to using the USB port, is to avoid corruption during an update. So if you plug into USB, then it is best to check everytime you turn off your TV that it is not updating the firmware first. The majority of the time, of course, it will not be, so it's not something to worry much over if you forget, but to avoid unnecessary problems, take those couple seconds to check.
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