If I recall correctly I did that because either the IIS SEO toolkit or google webmaster tools gave a warning about the SiteMap.aspx page as a page with no content. Pages with no content can hurt SEO, the SiteMap.aspx page has no content other than navigation links and search engine bots see it only like a page with a menu and no content. So for the same reason it wouldn't be a great idea to create empty cms pages with no content, it may be best if search bots don't look at SiteMap.aspx because the page is nothing but a menu with no real content of its own. Since the search engine bots should know how to crawl your site from your sitemap.ashx and from other pages that also have the menu links to crawl I don't think there is any SEO value to the SiteMap.aspx page, though of course you could edit the robots.txt file if you think otherwise.
It is strange if google is not crawling your site, you may need to contact google to find out why that is.
Other things I would look at is whether you are using any kind of IIS url rewriting rules that may negatively impact SEO or cause a bot not to crawl your site. Things like too many redirects going on can be a factor but I would expect google webmaster tools to give you warnings about it.