If it is a plugin you can delete php file (or the plugin folder) that provides the functionality.Wordpress will not even make an unnecessary check for the option.

The issue for me with plugins are these :
1/ You need to update these regularly. So you cannot leave your blog unadministered. You need to check regularly that there is no update and if there is one to update.
2/ Plugins are creating different setups and create conflicts. If you have a common core, as bigger as it is, it is a common denominator and every plugin writer has to cope with it. Right now, many plugins don’t work with some others because they cannot be tested and verified with all the setups, it is too time consuming (I fully understand this). So adding additionnal plugins could create additionnal burden…

If Wordpress wants to introduce features, the suggestion here is to introduce them as plugins.Not very different from options but the advantage is the user can completely discard them, if they are introduced as plugins.

So any person wanting to have a lighter wordpress can still have it without all those features provided by wordpress built plugins.Building it in the core as options, not only complicates the core and code modularity but it also becomes a necessary burden for everyone to carry (all those php files), though those code does not do anything for them, unless they turn on those options.

Is there any rule that wordpress should not maintain them if they provide the features as plugins? Wordpress can definitely make use of their own plugin architecture to indroduce features and continue to maintain them.

And that if they desire to provide features irrespective of whether or not they are already available via some existing plugins.

My thumb rule is that the core should have only code related to administration and security and those that are just enough to run a wordpress blog.

If you want to categorize plugins as a space only for third parties, then wordpress could come up with another seperate folder for files that provide functionalities other than that of core, namely administration and security.But that, if you ask me, is redundant.

I really prefer options…

Plugins are really for third party piece of codes. I mean, the core is the core. It is great to make it modular but keep in mind the incredible rythm of releases… This year, 2.5, 2.6 and 2.7. How many software editors release 3 major releases per year? Not a lot…

My feeling is also that integrating a plugin feature into the core is a way to acknowledge its effectiveness. Some of your plugins would definitely have their place in the core from my point of view…