Reasons to Restrict Multiclassing?

[Arkhios]

I see no reason to limit multi-classing. But then I think PHB+1 is a sign of haste to publish on the part of WOTC.

On related matter, I kinda agree on seeing no reason to limit multi-classing, except by recommending to think the "why" part of it.

However, I've come to understand that PHB+1 is purely because they want to reduce the amount of books from which each player draws their characters' abilities. PHB+1 keeps things relatively simple.
Plus, it's lesser books to have to carry around by each player, so that's a bonus, I guess. (I don't mind carrying a few kilos of extra. It's good exercise for otherwise very stationary hobby )