To Build Extensions Or Not

I started this topic mainly because I was wondering if it would be a good idea to build all possible components/classes as Yii extensions. The reason behind this is since we are suppose to write modular/reusable code, writing extensions will encourage, to some degree, improved re-usability. Not only is it easy to plug into other projects, you can share them as open source projects. Such cases can be seen in the jQuery community.

So what are your opinions on this? Please state your for and against reasons.

I think that extensions are needed because you can reuse your extensions in others apps.