The difference between having a portrait photographer do wedding pictures instead of a wedding photographer probably comes down to what you want. A portrait pro may want to shoot only in a studio or one suitable location, the result will be an excellent set of portraits but may not include any of the candids that make the results more interesting. A good wedding pro will take advantage of the locations at hand to give the images more context.
Also remember that there are currently a few different schools of thought on wedding photography, most prominent being traditional, photo-journalist and high fashion. The traditional style is the carefully posed portrait style, not very original but timeless since it does not give in to fads. Photo-journalistic tries to document the day, often capturing those fleeting moments candidly to give more meaning to the images, but it may look dated in future. High Fashion goes for artsier images which have a fashion magazine look but again may tend to look dated over time. I feel the best approach is to get the traditional portraits of the bridal party and them try out the other styles on the rest of the event.
Communication is the big issue, I would rather have an "adequate" photographer who will be easier to work with than a tempermental genius who puts the image ahead of the subject. A wedding is stressful enough without a pushy photog. Greg and mtngal have covered the important issues. I have seen people hire a professional videographer and have uncle Wally shoot the stills because he owned a good camera. To me this is ludicrous since an album of still images requires no real effort to pick up and view while watching a video can be a chore (not to mention a bore to others).
Just some thoughts, I havea wedding to shoot tomorrow. I will do mostly traditional poses because they are "safe" but I will shoot lots of aditional frames so that I have a record of the day.