it would not have been acceptable any other way.
I wonder what other LOTR faces we will see in the Hobbit? Elrond, Legolas, Gollum...many different roles can be reprised.
Also Galadriel, Saruman and the whole white council are busy behind the scenes in the hobbit, dealing with the Necromancer in Mirkwood, in Dol Guldur. Based on what was done with LOTR, the Hobbit might be rounded out with that bit of world-building.