This option turns on/off the span auto removal functionality. Unneeded span element will be removed for example <span class="someclass"><span class="someclass">content</span></span> will be turned into <span class="someclass">content</span>. This option is set to true by default.
Example of usage of the trim_span_elements option:
tinyMCE.init({
...
trim_span_elements : false
});