While pages are very interlinked, most people use ikiwiki for blogging. Blogging produces pages at random intervals and google apparently optimizes their crawls to fit the frequency of changes. For me it's not so often that the contents of my blog changes, so google indexes it quite infrequently. Sitemaps are polled more often than other content (if one exists) so it's lighter for the site and for search engines (yes, google) to frequently poll it instead. So it's not that pages can't be found, but it's lighter for the site to keep an up to date index.
On [Google Webmaster tools](https://www.google.com/webmasters/tools) you can substitute an RSS feed as a sitemap. Do not use Atom as if you have malformed XHTML it will fail to parse and you will get a ERROR message like so:
We were unable to read your Sitemap. It may contain an entry we are unable to recognize. Please validate your Sitemap before resubmitting.
[Google should grok feeds as sitemaps.](http://www.google.com/support/webmasters/bin/answer.py?answer=34654) Or rather [[plugins/inline]] should be improved to support the [sitemap protocol](http://sitemaps.org/protocol.php) natively.