The bride and groom are the centre of attention for the entire day so they must ensure that they include all their guests.
The parents of the bride and groom also play a key role. If the reception is being held at one of their houses, then a prominent role is usually assumed. At a hired venue, tradition usually dictates the the bride's parents 'host' the day.
Traditional receiving lines are not compulsory. They allow for the bride and groom, and the other hosts, to speak to all the guests, but they can take up a lot of the arrival drinks' time.