/etc/exports - a strange restriction

[Sergey Matveychuk]

Norbert Koch wrote:
>>/usr/tinderbox -ro -alldirs -maproot=0:0 localhost
>>
> As far as I know, it is a restriction of the nfs specification.
> If you only want to export a single directory, you could e.g.
> use mount_null/mount_nullfs(8).

Well, in my example remote host is localhost. But if I need to share 
files on a network?
I've made a quick look at RFC1813 but could not find this restriction. 
But why this restriction anyway for? It's very unconvenient.

-- 
Sem.