A wedding is a very special day and it is gone before you even realize it.
Some people planning a wedding try to exercise a modicum of frugality by trying to do as much of the work as possible and that is certainly a good idea in some cases.
In order to determine if skipping a wedding photographer and going the do-it-yourself route is a good idea or not, consider the following:

• To take good photos, you need the right equipment - Taking the best photos requires an investment in the right equipment.
  For pre-planned shots it may require portable lighting, reflective panels that make the best use of ambient light, a quality entry level or better DSLR camera, and a handful of lenses.
  Candid shots take even more equipment.
  The good news is that many parents already have the more expensive items such as the cameras, but a small investment in lenses and lighting will probably be highly advisable.
  
• You also need the right training - Taking the best possible photographs requires understanding what elements make a photograph good or bad.
  There is a lot of optical physics to understand, but the good news is that there are books on the subject and classes taught at almost every single private college in the country.
  Some photographers and artists offer independent classes and even tutoring may be possible, but if you do not know a focal length from depth of field, or have no idea what stopping up or down means, then it is time to hit the books.
  
• Experience is also required - Book learning is great for some people, but others will learn better by doing.
  Even those that have book learning will often need a lot of time taking pictures and a relatively thick skin to tolerate the input of other photographers regarding the technical faults in photographs.
  Do not despair, all budding photographers go through a phase where their pictures get picked apart by more experienced photographers as they learn what works and what does not work.
  Practice makes perfect and you do not want to be caught practicing at a weeding! This is the big leagues.
  
• Timing is everything - Do you always seem to find yourself at the right place at the right time? If so, it is probably not just luck.
  Instead, it might be a combination of intuition, experience, and social intelligence.
  Whatever you want to call it, being at the right place at the right time is what candid wedding photography is all about.
  

If you lack any of these key elements and cannot obtain them in time, then you should seriously reconsider hiring a professional wedding photographer.
Professional wedding photography is an art as much as it is a science and not everyone can do it properly.
You can still take pictures, but make sure the newlyweds' home has proper pictures to last a lifetime instead of photos that get hidden in scrapbooks and serve as a source of embarrassment for years to come.