15 minutes comes after several years of doing upgrades for 100+ sites.... practice makes perfect, they say... though honestly, sometimes it *can* go horribly wrong! Like I said, the longer you wait between, the more chance there is for a problem.
1) sometimes, especially if waiting many versions between upgrades. With the great new features of auto-updating, this has become easier and easier. Key to no problems with plugins is to use plugins by respected and supportive plugin developers. If they have a forum and the questions there are generally old and unanswered, DON'T USE THE PLUGIN.... same goes for the general information on the plugin with respect to the highest version supported... if it isn't up-to-date, don't use it.
I typically disable all plugins before upgrading... and frankly, I don't use that many, just key ones like get_custom, revision control, dagon's forms, SEO/XML and the occasional lightshow or slideshow, as needed.
2) Not really... because you are backing up your theme folder and its files... so after the upgrade, you simply restore, if needed. With 2.8x, I've used the auto-update and not had to restore those files at all.
But, if you customize core files or other includes or admin stuff, you might... for example, I DO a custom quicktags.js file for all sites as I prefer to avoid the wysiwyg formatting tools, building custom formatting tools for clients that are specific to their site. So, yes, I have to back up this file and restore it if the includes folder is modified by the version update. Same goes for any custom branding of the backend admin -- which is typically (for me) just mods of the images and such used in logins and at the top of the admin, etc.
So, yes -- if you count the time it takes to save and restore these files, it does add time, but I keep a text file of all custom files for each site, making it fairly easy to know what to backup and restore.
I have run into issues with old databases, though... when the mySQL db gets large, you can have issues with export/import of the tables, though mostly this has been a problem associated with FAlbum, imo.
3) NO, I don't upgrade EVERY time. I evaluate the reasons for the updates. If it is a security issue, you may only need to overwrite a single file... or more, but security issues are a KEY reason to do the upgrade. I don't always do the incremental minor upgrade (2.8.2->2.8.3, for example, but I would do a 2.8.2->2.8.4 because of security... or a 2.7.2->2.8.1 because of new features)
Security reasons motivate me; next are new and key features. And, of course, when WordPress radically changes how it handles categories or adds tags, etc... that simply makes sense to get on top of the changes asap.
HTH and remember, this is just MY reasoning... there may be valid reasons I don't know about or choose to ignore ;)