I can think of no condition in which this would mitigate XSS. 
If an attacker supplies &lt;script&gt;alert(1)&lt;/script&gt;  the result will be:
&lt;script&gt;alert(1)&lt;/script&gt;" which will still execute.  

Further more,  an attacker can just comment out the double quote in javascript:
alert(1)//"

You don't need an html comment to build valid html:

&lt;input name="search" type="text" value=""&gt;&lt;script&gt;alert('hi')&lt;/script&gt;&lt;input name="inject"&gt;


You should read the XSS prevention cheat sheet,  XSS is extremely common and you shouldn't come up with home-brew solutions to well known problems.

Is appending a double quote at the end of an input query a valid XSS prevention method?
What can be done with the MULTI-Flash feature of a 580ex II flash?  What are good situations to use this feature?
