Delayed commits make CouchDB significantly faster. They also open a one second window for data loss. In 0.9 and trunk, delayed commits are enabled by default and can be overridden with HTTP headers and an explicit API call to flush the write buffer. I suggest to turn off delayed commits by default and use the same overrides to enable it per request. A per-database option is possible, too.
One concern is developer workflow speed. The setting affects the test suite performance significantly. I'd opt to change couch.js to set the appropriate header to enable delayed commits for tests.
CouchDB should guarantee data safety first and speed second, with sensible overrides.