Most tag wikis (e.g. C#) repeat the excerpt in the body. This seems a bit repetitive when viewing the tag wiki page. Is it OK to edit the excerpt out of the body or is there some reason to leave it in?
Feel free to edit the tag wiki excerpt to boil down the essence of the tag and its concepts. Go for 120 characters or less if you want to.
Most of the excerpts you see are holdovers from when people were plugging away into the body but left little thought as to what to put in the excerpt of the tag.
There is no need to repeat yourself.
As the tag wiki is displayed separately from the excerpt, the information in the excerpt should also appear in the tag wiki body. For example, on the
/tags/NAME/info page, the excerpt is not shown; if the first sentence of the body assumes continuity with the excerpt, it will be incomprehensible.
This doesn't mean that the first paragraph of the body should repeat the excerpt verbatim; most of the time that is a bad idea. The excerpt needs to be short and to the point and cannot use any markup. The body can give more information and present it better.
To add to what reported in the other answers, the excerpts are now shown when a user adds a tag to a question.
The excerpts should report information that is immediately visible to who is adding the tag to a question, for example something similar to the following text:
Consider using [tag name] when the question is about [topic].
Such text doesn't needs to be repeated in the tag wiki, as that text is already read when a user adds the tag to a question, and that is the only moment that text is relevant.
Before, the excerpt was only visible in the page showing all the tags used in a site; it was not the text shown to who was adding a tag. It was only a short version of the tag wiki that was shown in a space where the full tag wiki would not have been visible.