Wanted by Felix Woldt as they use something similar already for their projects, discussed in issue #015359 ( http://issues.ez.no/015359 )

Basically, if you don't get a match, you should fallback to a match that matches host but not url. But the url part should be part of generated links so that you redirect the user to the match you want.

e.g. You have a set of siteaccesses that share content but differ by language with host_uri matches for:
example.com/de
example.com/en
example.com/fr

But when user enters just example.com, you might want him to end up on example.com/en.
This can currently be accomplished with apache rewrite rules, or config.php hacks, but it would be nicer that you could set all access related stuff in ezp it self.

The cleanest suggested approach so far (IMO) is to add a parameter on HostUriMatchMapItems to signal that url is optional, and thus if there is not a match on url we still consider it a match.
Only user needs to take care of is make sure this setting is only used on last match for example.com