/%postname%.html this could be critical when you have a link after the postlink.
example your post is called nubiles and you have another link to your blog on
http://www.yoursite.com/nubiles.html. maybe this link is called "teenslut" then wordpress generates it to
http://www.yoursite.com/nubiles.html/teenslut/
and it should be a 404.
when you use /%postname%/ it looks like
http://www.yoursite.com/nubiles/teen/ which looks better and its much more se friendly then the html ending