But I regard any plugin that does only trivial things (which can be easily done manually) as unnecessary. If I had to use a special plugin for every tiny change (e.g. for adding new sidebar/footer links), it would drive me crazy.

To each, his own of course; but I'm exactly the opposite. I'd rather have 60 single-function Plugins active than one Plugin that does 60 different things.

I usually tend to check whether the plugin is safe to use – doing that every time a new update is available would be insane. Not to mention dealing with potential incompatibility issues, poorly written code etc – that’s really something I’d want to avoid.

The ease of verifying the safety/integrity of a Plugin update is inversely proportional to the complexity of the Plugin.

I don’t switch themes very often anyway. Besides, tweaking my own theme makes me happy. :)

If you're in complete control of your own Theme, then it makes little difference whether your functionality code resides in Theme or Plugin. But if you're not in complete control of the Theme, then you're just setting yourself up for problems.

My personal preference is a site functionality Plugin, that separates out different functionality into different sub-files. It makes troubleshooting as simple as commenting out the include() call for a given sub-file.