When looking for a speaker for an event, in the vast majority of cases a professional speaker is the best option by far. Occasionally, the need for expertise in a specialized field may dictate that you have a non-professional speaker with the requisite expertise, but this is rare. From the many advantages of hiring a professional speaker, consider the following:
Large pool of talent
The vast number of speakers who make a living from speaking engagements makes it more likely than not that you'll find a speaker who can enhance your particular event. Some speakers have a wide range of speaking topics; others may specialize in a particular field. Agencies which hire out professional speakers are experts in recommending suitable prospects. You'll also have the choice of high profile speakers who can attract audiences and publicity, or those who are skilled in motivational speaking, fund raising or industry-specific presentations. Your choices from willing amateurs are much more limited.
You can expect a professional to have good techniques - speaking tone, volume, pace, posture. They can use microphones and audiovisual equipment, and should be adept at handling questions and audience interruptions. A professional can speak on the agreed topic for the agreed time. You can specify the type of presentation you need - facilitating a workshop, making a keynote speech, an educational presentation, whether notes or power point slides are to be used. An amateur speaker may have an interesting topic, but the delivery is even more important - if you can't hear the speaker, or if you're suffering slow death by power point, then even the best topic in the world won't save the situation.
When you hire a professional, then professionalism is what you can expect - well presented, punctual, someone who's done their homework and knows his or her audience and topic, and who will use humor appropriately. There are few situations more uncomfortable or cringe-worthy than listening to someone pretending to be an expert, or who tries to be funny but isn't.
You'll obviously feel more comfortable about the presentation if you know what to expect from your speaker. One great thing about hiring a professional speaker is that you're likely to be able to get references from previous engagements and maybe even watch an earlier performance online or on disc.
You get what you pay for
When you're paying a professional, you get to call the shots, telling him or her exactly what you want from the presentation. If you have a volunteer or a friend of a friend, you're more beholden, and may feel that you have to let the speaker decide, as if they're doing you a favor, which is exactly what they are doing.
Probably, the greatest advantage of hiring a good speaker is the connection he or she makes to the audience. It boils down to a combination of factors such as conversational delivery, telling a story rather than reading from notes, displaying an enthusiasm for the topic that's contagious and opening up a part of yourself to the audience. As contradictory as it may seem, speaking in such a natural manner is something that actually takes considerable skill and experience, and that's why you owe it to your organization to engage a professional speaker for your function.