You're right about the small segment of multiplayer cards, but the lack of rules for multiplayer results in some...issues. In team games, for instance, I've always been bothered by the definition of 'opponent' with cards like Animal Magnetism, where you want your teammate to still count as an opponent, and Screeching Buzzard, where you don't want them to be 'opponents'.
Even if we don't get a larger segment of multiplayer cards in each expansion (draft and limited formats may keep the number from increasing) having formalized rules will help them to design better multiplayer cards, designed with the full power of rules designed for them. For example, the current batch of rules is naming teammates 'teammates' and not 'opponents', which will allow the developers to make teammate affecting cards they couldn't do before since the word teammate wouldn't have meant anything in a rules sense.
You're right that in some ways having formalized multiplayer rules might make things more complicated, but on the other hand, you could also argue that nothing is more complicated than needing a rules clarification that there just aren't rules on. Also, if you're concerned about the end product, Wizards is also giving us ample opportunity to assist in giving them feedback on their work in this case.
Incidentally, it will also allow for multiplayer tournaments, but I'm not sure that I'd be overly excited about that if the tournament scene develops in the same direction as two-player tournaments have. An Emporer draft tournament sounds somewhat enticing, though...at least when they said that they had team tournament formats at Prereleases, this will pave the way for actually having them available instead of those fake-out team games they have now.