Why don't we hack core? I couldn't believe that this question hasn't already been answered on this site, but I didn't find it when I searched, so...

Why is it such a bad idea crime against nature to hack core?


Is it really that great to be able to upgrade your core version? Most of my sites end up having horribly out of date cores anyway, so why bother? 
Even if it is so bad for site owners, why does the community care so much? Why is it referred to as "killing kittens"? Isn't that rather hyperbolic?
Hacking core is so easy, don't we like taking the easier route to a problems' solution?
Aren't there problems that can only be solved by hacking core? What then?
I could write a massive answer here, but I'm just gonna post this link: Never hack core!

The main reason I guess is that if you hack core to do something you need, and then update it...BANG!  Your changes are gone. Lost. You could then try and roll back the code from your VCS, but seeing as you can't roll back database upgrades from Drupal core - you're looking at restoring all code from VCS, and then restoring databases from your backups. All the time you're trying to roll back your code, you'll probably then notice that your last pre-update database backup failed, and you'll swear more than you've ever sworn before.

Also - most importantly - if you hack core, then Dries and Webchick both kill a kitten :-o