It is extremely important.
Asking a friend to be your MC is a poor idea. Once you ask a friend to be MC, they are no longer a guest at your wedding, they are an employee. The MC role requires meticulous attention for the entire evening, constantly liasing with the band, photo booth, venue manager, caterer, the bride and groom, their guests, the photographer, the videographer etc. When this careful attention isn't paid, it shows; and the reception is simply not up to standard. As a professional wedding MC I set the standards extremely high, your wedding deserves absolutely nothing less. In my opinion, is must be perfect, you have invested a lot of time, effort and money into your big day...your wedding is worthy of being exactly how you have planned/wanted it. Additionaly, yes your friend will put in their best efforts, but their lack of experience means they don't know what they don't know! No one would ever doubt their effort, but unfortunately you can't instantly become a great MC by doing a google search or YouTube, it take years of honing the MC craft to become a professional. Often when google or YouTube is used, your wedding is a carbon copy of someone elses. A good MC will have an indepth questionaire that they will provide you prior to your planning meeting together, this personalises the wedding and makes it specfic and unique to you.
People often think that someone who "is a good talker" will make a good MC, this isn't neccasarily true. around 20% of what an MC does it on the microphone, the remaing 80% is the planning prior and the execution on the night. I won't list all the tasks, duties and responsibilities of what an MC does on the evening, becuase the list is enourmous. It is a constant effort of liasing with all parties involved in the wedding to ensure that it runs like a well oiled machine.
I could go on for hours about how important a professional MC is! But that's probably enough for now. If you have any additional questions please feel free to make contact, more than happy to help out.