Horses: Pros and Cons of being shoed or not?

This is just a question I thought of earlier today when a customer asked me about my horse and how she was doing. He asked me about how much it costs to shoe her and was very dismayed to find out we don't shoe our horses and do the proper daily maintenance (trimming, cleaning) ourselves. He started talking about the problems with not shoeing them but his only arguement seemed to work in favor in either way so can someone please tell me the pros and cons of shoeing your horses. I've always thought that as long as you proceed with good maintenance and be weary of where your riding and keeping riding on the road to the minimum that it was alright. SO anyones knowledge would be great, I'm a first time owner but have been around horses most of my life so you would think I would know this -.-;;