Having Barney marry Robbin makes the entire series at least have a point to it. If it's Robin's wedding where Ted meets the Mother, then it makes some kind of sense to have started the entire story of 'how I met your mother' from the day he met Robin. Otherwise, why in the world has he been telling this story from years before he met the Mother?
I've been totally in love with this show since Episode 1 and I'm torn as to weather we should meet the mother a year before the show ends (which would actually be kinda confusing and probably make them loose viewers for the final year) or on the last episode. A happy comprimise would probably be a hour or 90 minute long finale that shows them meeting and then a quick flash through their relationship, wedding, and the rest of thier lives.