Okay, not all instances of Facebook unfriending are a bad idea. If you are concerned about security, or confidentiality, or the person is regularly being an arse on your threads, or if there are past romantic links, or if they've really hurt you deeply in a way that is beyond repair, then yes, there are conditions under which unfriending is probably advisable.
But it has to be said, those cases are the minority of cases for unfriending. In the majority of cases when people unfriend, they do so for reasons that when examined more closely turn out to be quite illogical. Off the top of my head, here are the most popular reasons I think people unfriend Facebook friends, in no particular order:
1) They are self-absorbed pests who keep clogging up your newsfeed with annoying or irrelevant posts several times a day.
2) They keep making rude or annoying comments on your posts.
3) The person has views and beliefs that you find ignorant, odious or down right repugnant.
4) They are part of a religious group or political group you dislike and want nothing to do with.
5) You have a cull and unfriend all the people you don't really know, or know but don't keep in touch with.
6) You sense you've amassed too many friends to keep up with, and even though they have value to you, you figure that your list should be reduced to just the most important few dozen or few hundred people.
7) It's bad for your brain to have too many people in your social circle - and Robin Dunbar, he of the Dunbar number fame - the theory that there are putative cognitive limits to the number of people with whom folk can maintain stable social relationships - believes this applies to social media too.
8) They keep sending annoying requests your way - game requests being the worst, as we all know.
Despite being popular reasons for unfriending people, I think I can make a good case for why people may want to think twice about doing so for those reasons. Let's take them in turn.
Number 1, they are self-absorbed pests who keep clogging up your newsfeed with annoying or irrelevant posts several times a day. Simple answer: unfollow them so their posts no longer appear on your newsfeed. They will become invisible to you except when you choose to check out their activity every now and then.
What about number 2: they keep making rude or annoying comments on your posts? You have two choices, neither of which involves unfriending. You can ignore them - after all, even though they're posting annoying comments on your posts, they are at least getting to read your views, which might plant seeds of good in their mind for later. Plus, even annoying people may sometimes make observations that are worth reading. Or if that doesn't appeal you can hide your posts from their newsfeed so they can no longer comment.
Number 3 and number 4 amount to a similar problem, and therefore have the same solution. Those who have views and beliefs that you find unpleasant, and/or are part of a religious group or political group you dislike and want nothing to do with can simply be unfollowed. Unfollow them but don't hide your posts from them - that way, you spare yourself their unpleasing posts but they get to see yours and hopefully may learn something or benefit from what you have to say.
Numbers 5, 6 and 7 are also quite similar in nature, so again the solution applies to all three cases. Just because you have lots of friends that you don't know well or keep in touch with, that again doesn't mean you need to unfriend anyone. Simply fix your settings so you only see the posts of people you want to see, but don't hide your posts from anyone who isn't guilty of number 2 above. You decided to be friends with them in the first place, so there must have been a time when you saw value in having weak tie friendships on Facebook. Customised groups and privacy settings are useful, and you can even set up close friends on Facebook to ensure that select posts, or most posts if you prefer, are shared with only a very specially chosen few if that's what you want.
As for number 8, that's really easy - block all requests from them, then apply all the above to whatever situation remains.
With the ability to hide anything you don't want to see from others, and hide from others anything of yours you don’t want them to see, unfriending someone seems like something a lot of people do rather too hastily, particularly bearing in mind that those who are not of your mindset or to your liking needn't necessarily be robbed of the ability to read the copious amounts of wit and wisdom you put out there.
Except for those extreme circumstances I mentioned, like the person is Rolf Harris or something, when you unfriend someone, what you're basically saying is, your online significance is so unimportant to my life that I won't even allow you to be a forgotten entity hidden from my newsfeed, and me from yours - not even qualifying for the 'You can be a number on my friends list but totally off my radar in every other way' status.