TinyMCE can be added to a Magento site. Magento allows for creating files in a namespace that is not overwritten by upgrades. As a developer, one is supposed to place custom theme changes in his or her custom theme. For example, custom modules should go into app/etc/modules. Custom code should go into app/code/local. Custom frontend template changes should go in app/design/frontend/default/mythemename/templates/, and so on and so forth.
Surely, if you hack the files that come with the Magento installation, you will lose your hard work during an upgrade, or spend precious time merging your changes.
I have written a step by step tutorial for adding TinyMCE to a Magento site the proper way, with all source code and explanations included. This method does not change any of the Magento files. You can see the Wiki at
http://www.magentocommerce.com/wiki/wysiwyg/tinymce_wysiwyg_the_magento_way .
Also, this version has plenty of plugins for a feature rich button bar.