Hiring a professional event photographer is a great move if you have an important event to document. After you've worked hard and spent a great deal of money on putting your event together, hiring a pro makes sense because once the event is over the photography is the most important record that you have to tell the story.
Images are a critical part of your organization's presence at a time when the world knows your organization through its web site. People too busy to read articles will always look at the pictures, especially if they are great pictures!
Why Not Use Amateurs?
You can get some good, free shots by using amateurs who volunteer their time. It's a good way to save money, but you may be saving money in the worst possible place given the importance of the images. There are things that pros can do for you that a well meaning hobbyist can't do for you.
TIME. Most amateur photographers cannot or will not spend the time editing hundreds or thousands of images, adjusting them for cropping, color balance, brightness and contrast, etc.. It takes longer to edit an event and put together the story than it takes to photograph it, and many amateurs are likely to simply make a photo dump--send you a boat load of images that are not a final product that you'll want to use to show the world who you are.
SKILL LEVEL. There are great amateur snap shooters in the world, but their skills levels are unlikely to be at the same level as someone who does the work full time, and whose ability to pay the rent depends on whether they get the shot or not. Practice may not really make you perfect, but it makes you much, much better.
ALL WEATHER SHOOTING. If it storms on your event, your volunteer with the nice camera will most likely take shelter to protect their expensive equipment. Professionals will have rain gear for themselves and their equipment. Rain shots can be dramatic and beautiful, and will tell the story of the people who care enough to brave the elements to participate in your event. Some of us are happy when it rains.
BACKUP EQUIPMENT. Professional camera equipment is expensive. Most amateurs can't foot the bill for multiple cameras, lenses, flashes, etc.. If their equipment goes down for any reason at your event, you may not get pictures. Professionals carry backups for everything; they just can't afford not to given the implications of their businesses if they fail to produce.
LIABILITY. Do you want to deal with the liability if someone trips over your hobbyist's camera bag and is hurt? Professional photographers have liability insurance to cover such things.