DOMParser Vs ResponseXML?

In processing the response from an AJAX call, assume the returned xml string as:

"<?xml version="1.0" encoding="UTF-8"?>
<options>
<option text="new search..." value="search..."/>[code].....

Now, in the javascript:

1)If I use ajaxRequest.responseXML, I can directly use it as DOM document, such as: ajaxRequest.responseXML.getElemetnByTagName("option");

2) Or, I can use another way(assume on Firefox):

var parser=new DOMParser();
doc=parser.parseFromString(ajaxRequest.responseText,"text/xml");
doc.getElemetnByTagName("option");

My questions are.Since I can use ajaxRequest.responseXML.getElemetnByTagName("option") directly, I do not need DOMParser to convert.On the other hand, if the ajaxRequest.responseText is NOT of the XML format, the parser.parseFromString will not work any way.Then, why (under what circumstance) should one use DOMParser?