I’m awed that my original note—sparked by confusion over “Reachability” vs. force-quit; I’ve turned off Reachability and everything is now as (I assert) it should be on my new SE—has inspired so much discussion.
On my iPhone6 which runs the latest available update I have problems with Messages. It shows there is a new message but it will not allow me to access it, so I shut down the phone and, upon restart, it allows me to access the new message. This happens almost daily.
When I save up some money, I will get me a new phone…
Until then I will live with the short battery life (original battery) and cracked screen!
The battery in my (old model) iPhone SE was very weak before I had it replaced. I would sometimes turn the phone off completely to make it last a little longer if I did not need to use it for a few hours. Probably turning it back on was more of a drain than some amount of normal use. It did sometimes seem to give me a little more life, but I’m not sure of that.
As with all things, “it depends”. Many of the big power-drains (e.g. the display) are powered down when the phone is on standby. But others (e.g. the cellular radio) remain operational.
The power consumed by the cellular radio depends enormously on your location, relative to your local cell towers. If you’re near one and have a clean signal, your phone will reduce the radio power (transmit power and receiver gain) in order to conserve the battery. But if you’re far from one, where there is a weak signal, or completely out of range, then the phone will boost the radio power to its maximum, to try and lock onto whatever signal it can find (with the assumption that losing connectivity is worse than battery drain).
Of course, a power-on sequence does use a lot of power, but that’s relatively brief. Once the OS is fully booted and the cellular and Wi-Fi radios have locked on to their respective base stations, power consumption should drop back to its normal background levels.
So how long must it be off before the gains from being off are greater than the losses from a cold boot? I couldn’t say, but I think your criteria (“a few hours”) sounds reasonable. And if you’re in a place with a weak cellular signal, then it might be even less than that.
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I do live in a place with a very weak cell signal, for which reason I keep the phone in airplane mode when I’m home. I still do that even now that I have a good battery. I’ve tried but so far failed to automate that using Automations, which seem to be pretty lame.
I don’t think anyone has mentioned it but back in the early iPhone days before unlimited data plans, I would regularly force quit apps because some apps would continue to use data in the background (e.g. Google Maps). On more than one occasion I blew through my data plan cap when I inadvertently left Google Maps “alive” overnight. I haven’t experienced any misbehaving apps in years so now I only force quit apps periodically (every few weeks?). Unfortunately I impressed on my wife the need to force quit apps back when it was an issue and now she absolutely refuses to change her behavior 
That is a very real concern, but nowadays there’s no need to force kill apps to prevent that. Just check Settings > General > Background App Refresh. I have almost everything turned off in there except for a few well behaved apps where it makes sense to have them keep updating.
Reading the above referenced documentation (from Apple) doesn’t clarify the situation to me.
“…
2. Swipe right or left to find the app that you want to close.
3. Swipe up on the app’s preview to close the app.
When your recently used apps appear, the apps aren’t open, but they’re in standby mode to help you (…). You should force an app to close only if it’s unresponsive.”
One the one hand this clearly says “to close the app”, on the other it says do this “only if it’s unresponsive.” But, why? And, what does ‘standby mode’ mean? Are we sure Facebook isn’t still tracking your activity/location etc. even when in standby mode?
I use many apps and find the App Switcher becomes crowded and inefficient if I leave all the apps “in standby mode” so I swipe up to close the ones I use infrequently. The referenced Apple webpage doesn’t mention any downside to ‘closing’ apps even if they’re still responsive.
So, what is the downside to closing apps that are used infrequently, if there is one?
That’s up to you. As I wrote above, check Settings > General > Background App Refresh.
As discussed above, additional power draw and wait time when you have to relaunch them from scratch.
It’s a bit unfortunate, Apple chose “close”. It’s more like force quit. The OS does it, the app does not run or get an opportunity to save anything. It’s nuked by the OS. On the Mac, the equivalent is force quit vs. quit. What Apple here calls “close” is force quit.
As I said initially and others have repeated, battery life is reduced because it takes more juice to launch an app that has been forced to quit. It also waste your time waiting for it to launch, if that’s important to you.
On this page; Switch apps on your iPhone, iPad, or iPod touch - Apple Support
If you quit an app from the app switcher, it might not be able to run or check for new content before you open it again.
Maybe this doesn’t matter to you, but if you have, say, a podcast player app that is supposed to download new episodes in the background, it may not be able to if you force-close the app.
In addition to what others have written, another downside is that you’re wasting your own time. :)
The apps you see in the App Switcher is recently used apps. They aren’t necessarily open or doing anything. So flicking away them away is, at worst, disrupting the app, and at best, engaging you in a joyless game of whack-a-mole.
The other reason I find myself force quitting is when I don’t understand what the app is doing. When my Music app drained my battery overnight last night, I force quit it after reviewing the Battery chart and find the culprit. I think this and freezing is the only reason I force quit, but even with a “clean” app like Music, if it is doing something that I don’t like and don’t understand, I turn it off.