I just came across this after ceremony activity on TK planner. I hadn't thought of it before, but they make a couple great points for having one:
1) A receiving line is the best opportunity to greet each guest  individually and thank him or her for coming to your wedding. And if  you're having more than 50 guests, it's considered proper etiquette. The  line also guarantees your guests a minute of face-to-face time with  you, a chance to hug, kiss, and congratulate you both, and to say things  like "The ceremony was lovely." If you rely instead on the more casual  greet-them-as-you-see-them approach, you may spend the whole party in a  tailspin, ducking out of conversations to say hello to people you  haven't greeted yet, and inevitably you'll end up missing someone. 
Read more:  
Receiving Line: Etiquette, Options & TipsTheKnot.com - http://wedding.theknot.com/wedding-planning/wedding-customs/articles/receiving-line-etiquette-options-tips.aspx#ixzz1yFH8oRs32) The receiving line is where your hosting duties as the bride and groom  kick off. It'll no doubt be a whirlwind of faces, but as much as  possible you should introduce your new spouse and your parents to all  the guests they have not yet met. First names and the guests'  relationships to you should suffice. Likewise all guests should take it  upon themselves to offer this same information as introduction to  attendants and family members whom they've never met as they proceed  down the line; simply shake hands, offer congratulations, and keep  moving. The bride and groom need only accept everyone's hugs, kisses,  and best wishes, and thank them for coming. It's that simple. And yes,  you'll end up with a lot of lipstick on your cheeks, but fear not --  you're allowed to make a bathroom pit stop before heading to the party.