According to this:
http://forum.naruto.oasgames.com/en/forum.php?mod=viewthread&tid=20255&extra=page%3D2
It's against the rules if it benefit the main account.

As far as GNW goes, it's a bit complicated.
Now, first and foremost, if the alt made a difference in the outcome of the war, then it would have indeed benefited the main account--in the very least you get 13 extra contribution for winning a round instead of losing. Also, higher ranking will mean more reward per person in terms of packs, so might mildly increase reward there.

Baring that, even if there is no difference in outcome of the war, it does help in increasing group fund which in turn does benefit all members slightly. But that's probably too tiny for anyone to care.