The fields uses either <p> or no tags at all. It doesn't use only <br> tag.
I've just tested with Themer's text module and it seems to always output <p> tag, no matter what the data is. I'm wondering why don't you use the <p> tag.
In the output I'm trying to create, I have a line of static text followed by the output from a WYSIWYG field. <p> provides significantly more spacing than <br> and in this specific application it looks bad. What I'm trying to output is <p>my static text<br>first line of WYSIWYG output text<br>second line of WYSIWYG output text</p>