I've heard this time and time again from many of my clients, "We have a friend that DJ's on the side sometimes" or, "Can't we just set up our own playlist and play it through a speaker?". I cringe every time someone brings this up.
What many potential clients don't really think about is the training and professionalism it takes to be a disc jockey, especially for a wedding. There's a lot more to a wedding reception than pressing the play button. Consider the planning aspect of the event, such as when is the first dance and when the cake should be cut and so on. Also consider this, does your friend who "DJ's on the side sometimes" have a wireless lapel microphone for your officiant or does he/she have a wireless handheld microphone for speeches and toasts? Certainly a playlist and a speaker won't accommodate toasts without a microphone. Has your DJ been trained as an MC? Professional DJ's and MC's do obtain voice and microphone training from reputable sources. These are just basic examples.
Have you ever thought about how your guests will be entertained during cocktail hour or dinner? I'm pretty sure you're going to want more than Kenny G. and Michael Buble tunes playing in the background. You're going to want you guests to feel not only excited about the party that's about to happen, but you want them to be energized and ready to fill the dance floor! Most couples want a professional DJ who knows how to motivate your guests to dance and get them involved in the reception and it takes a pro with tons of experience to pull this off.
Your DJ should know how to "read" a crowd and play the music that not only you as a client will respond to, but your guests will as well. A well-rounded disc jockey knows more than just one genre of music. He can go from country to pop to oldies to hip hop to 90's dance and back to country without skipping a beat. He will learn very quickly what kind of music you and most of your guests will dance to that evening. A set playlist can not accomplish this and most people who "DJ on the side" can't either. You're going to want someone who has nearly an unlimited selection of music that was purchased, not pirated and has a digital bit rate comparable to that of CD quality sound.
Speaking of sound, professional disc jockeys spend literally thousands and thousands of dollars purchasing PA equipment that not only sounds incredible, but will also stand up to the rigors of transporting, setting up and tearing down several times a week. Professional DJ's always carry back-up components with them in case something happens in the middle of your event. I carry a back-up computer, hard drive with music, microphone/batteries and speakers...just in case something happens.
In conclusion, yes you need a professional, full time DJ for your event.