The standard single monument is a tribute to the uniqueness of the individual.
Single headstones generally allow for longer inscriptions. A single stone spares the surviving spouse the awkward feeling created by the not-yet inscribed, second half of the monument. A corresponding single stone can be replicated for the spouse in the future, to match the original.
(They can be built to identical specifications of a deceased family member's monument, or can be totally different if so decided.)
It is important to bear in mind though, that when choosing a full monument consideration needs to be given to the possibility of the requirement of a double monument in the future. Due to the construction methods a double monument is more cost effective than two single monuments.