The problem occurs because, for instance, index.shtml won't delete and replace index.html. Just as you can have the two files in the same folder on your home computer at the same time, the OP in that thread stated she had both the html and shtml versions up at the same time. Having different names, they will both coexist quite nicely, which is why one has to go in and delete the .html pages or the problem of double content occurs.

Basically, without the 301 redirect, Google and other search engines will not know the pages were deleted and replaced with new differently named pages. Instead, the .shtml pages would just be indexed as new pages that don't have ranks to inherit.

Thanks carey, That made a bit more sense to me.

oh bugger! LOL

Thanks guys for clearing up that issue, I guess in the long run I'll have to take some time to figure out the 301 redirects so I can utilize this system at some point.

Yea, .shtml is worth it if you can either do the redirect or don't mind starting over with the page rank.