I agree with you. I only submitted my site's sitemap to Google Webmaster Tools, and it helps me to rank better.
if you want to increase your pr,i think you should go to get more and more high quality backlink and keep on update the content.
I think there is no need to submit a site map since your site is already a very old one. Yet nor will the submission affect your PR. It is neither helpful nor harmful. You can just ignore it, IMHO. You can get your site indexed by getting more backlinks.