gazouteastAndrea
To use an analgous example based upon my thinking that you're located in the UK or Eire - Imagine creating a site to cover .... errr .... caravan parks Europe wide.
You have a blog for each country in its own language, specific to the parks in that country. A user from Yugoslavia spots the yugoslav blog and wants to join so they can get into the members area to get the discount codes from the blog author for the Yugoslav parks ... but ...
... then they get bounced to the site home blog to register - with a form in English (a language they don't speak, let alone read or write) and what do they then do? Popular answer would be to say "Oh stuff this" (in Yugoslavian of course) and they then bad-mouth the site amongst all their friends believing the Slav language blog is just a baited hook to get them to join some other site that they cannot read and have no idea what it will try to sell to them.
Can you see the issue now?
When child-blogs tout for members, in the main, interested readers want to sign up directly to the child-blog they are reading - they are likely not techies and developers. They don't care and don't want to know about parent sites and stuff - they just want to join the blog they are reading.
If you want to look at it as a techie issue - consioder BuddyPress - why have limited membership groups? Why have groups that you can join exclusively without being a member of the other groups automatically? The same reasons apply to the blogs. Maybe the site admin does not want a new child-blog member to automatically be a member of other child-blogs - whether for privacy, or language, or other reasons. This is purely and simply a customer service point from the angle of WPMU developers. The customers want this, therefore give it to them.
;)
Gaz
