DIY can be great fun, however on one of your biggest and most important days of your life why put yourself or family and friends through the stress of setting up your ceremony. Here are some very valid reasons:
1. Buying your own arbour may sound easy enough however getting someone to install it on the day can work out stressful and detrimental. Often people don't realise the time required to set an arbour up and underestimate this. Which then leads to a job half done and rushed and perhaps not what the bride and groom were expecting. Not only do you have to purchase an arbour (which can cost a lot) you also have to decorate it and again this can cost a fortune once you have bought flowers and fabric...etc. It is not a cheap exercise by any means.
2. Aspect - After years of setting up wedding ceremonies, I certainly know how to enhance an environment and place furniture and equipment to suit the chosen location the best. And style it perfectly so it looks absolutely beautiful. I don't do tacky, its all class and quality.
3. Lighting - Shadows and couples looking into the sun is something a lot of people don't think about however with all my experience I can suggest best set up to allow for the best light and photos. Also not having the couple squinting for the photos is something very important! (if possible - sometimes it cannot be helped).
4. Photos - having your set up looking its absolute best is crucial to secure the best possible photos. Most weddings are conducted at 3-4pm so you have the best light and conditions.
5. Packing down - who would seriously be in charge of packing down a ceremony setup after celebrating a wedding with friends and family? Heck like no one, everyone would much rather be in the moment and toasting a glass straight away after a wedding right!
This is just a small list of many things that I could say, but it certainly gives you an idea of what is required from a professionals point of view.