There are about a MILLION blog posts out there that go over this same topic, but sometimes it doesn't really matter unless it comes from someone you personally know, so I thought I would share this. (:
WHY does your photographer get so excited if you say you're having an unplugged wedding?! Because it simply makes our lives about a million times easier. We don't have to worry about missing SUPER important shots! Not only can other cameras/phones/tablets/etc. block us from getting important details - but the flash can also cause LOTS of problems as well!
We set our camera settings to get the best shot at a perfect exposure! When another flash is going off, this changes where our settings need to be and tend to blow out/wash out your images!
Luckily, I've never had this problem during a ceremony - but here's a quick little example of what would happen to your photos during a ceremony if there is flash interference from another camera.
These images were taken at a fun time during a reception - only a couple of seconds apart from each other. The first one, a guest had triggered their camera at the same time causing their flash to go off at the same time as me, on the right, is a properly exposed image that I took a couple seconds afterwards.
Sometimes, we can try to save these images but most of the time the extra flash blows the image out far past saving - this would be horrible if it was during a super important part of the wedding, such as any point in the ceremony, first kiss, etc.
Now, don't get me wrong - I encourage guest participation during the reception! But please remember to be courteous of the photographer at these events - the Bride & Groom pay us to be there and capture these moments as-is for them. So please during the ceremony and important parts of the wedding, put your cameras away & embrace the moment! (: