1. Light light light
As I mentioned above, many wedding ceremonies are scheduled for the evening, around dinner time, and depending on what time of year it is or where your ceremony is taking place, you could be stuck trying to get your portraits done after dark. I'll tell you, it's absolutely possible to do that, but when you look at most photographers' portfolios, including mine, the photos you see are almost entirely shot in daylight or evening sunset light. Night photography will give you a totally different look, so if you're after the type of photos you're probably hiring your photographer to create (i.e. ones similar to what you see in their portfolio), you probably won't get that look. I love shooting portraits before the ceremony and then also grabbing you two for 10-ish minutes and dipping out of the reception for a quick sunset session.
2. Time is (not) on your side
Wedding days are kind of crazy. You're trying to fit in a bunch of stuff all on one day, and one of those things is photos. If you don't utilize time wisely, you might start to feel the time crunch, which is more stress, and we want less stress. Wedding days fly by. Like, as super sonic speed. It's insane. I remember getting to the hotel after my wedding and I felt like I'd just been through some crazy whirlwind. It'll probably feel that way no matter what, but you can create a schedule that avoids the stressful part of whirlwinds.
If you wait until after the ceremony to get all of your portraits done, you're essentially only utilizing 50% of the time available on the day. Take advantage of that first part of the day and get your photos done. It'll be one less (rather big) thing to stress about.
3. You want your family photos with both of you right?
Time is at a premium on a wedding day, and family photos are usually a must-have on the list of photos. Getting these done before the ceremony means two things: first, it's way easier to wrangle all your relatives before all the other guests arrive. Your super social, beer loving uncle will probably make a bee line for the bar and group of guests to entertain right after the ceremony, along with plenty of other family members who conveniently didn't hear the instructions to gather for family photos. That means scrambling to get everyone present for the photos you want. Second, family photos take a considerable amount of time. There are always multiple groupings, which can get even more complex when there are divorces, separations, and step families, and everyone has to wait around until all the photos are done, which means they're not enjoying the amazing party you've spent months or years planning and putting together.
The other option, when the time crunch is just too much, is trying to do family portraits before the ceremony with only one of each of you. You could do the bride and her family and then the groom and his family separately. But, then, one of the best parts of a wedding is the bringing together of two families through your guys' union, and not having both of you present kind of seems silly.