XMLHttpRequest overrideMimeType question..

hope this is the best place to get some input on this subject...

have seen the mass use of overrideMimeType('text/xml') when using the 
XMLHttpRequest object.

I would really like to know WHY this seems to be some magic "band-aid" 
for some peoples code, esp when they say it fixes many calls to even 
HTML pages, that they would possibly never be a "well-formed" XMLDocument??

any thoughts will be appreciated, &/or if another group would get better 
results?