i've seen tonnes of marriages in india [both in urban and village settings] where an ordinary person solemnized the marriage coz he was knowledgeable and had necessary documentation...imaam is NOT even the right word...it used to be called the Qaazii. :)
i'm not saying you're wrong, you're absolutely right, you dont have to be an imam to conduct the marriage ceremony but people ask an imam (and I mean an imam, not a qazi) to do it because the marriage will be legally recognized then as well as religiously.