While Roman Catholic theology (as opposed to Eastern theology) holds that the ministers of the sacrament are the couple themselves, the Church requires a priest or deacon to officiate as the official witness on behalf of the Church for a valid and licit sacramental wedding.

Otherwise, two Catholics could get married by anyone with a secular licence and have that marriage accepted as a sacramental marriage. In fact, if two Catholics got married in this fashion "outside the Church" and then divorced and each spouse sought an annulment to remarry in the Church, they would be told that they do not required annulments because their first marriage was not valid. 

Your concern appears to be belittling the "ordained" diaconate. I am not engaged in that discussion. The significance of the diaconate as an ordained ministry within Catholic canon law is not dependent on what deacon may or may not do liturgically. A deacon is a cleric in canon law,  and there are many things only clerics can do.