The Chizkuni (Shemot 12:18) says thatalthough there is no obligation to eat matza the entire week of Pesach, one who does eat matza fulfills a mitzva by doing so. He explains that when it comes to most mitzvot, one not only merits reward for observing them, but also deserves punishment if he is negligent and fails to fulfill them. However, some mitzvot (such as eating matza after the first night of Pesach) do not entail any punishments but do bring reward to those who opt to fulfill them.
It is well known that the Gaon of Vilna maintained that eating matza all Pesach is an optional mitzva. It is related (Ma’aseh Rav 175) that he accorded immense value to this mitzva. In fact, he would make a point of eating se’uda shelishit on the last day of Pesach (although he did not usually eat se’uda shelishit on Yom Tov), in order to fulfill the mitzva of eating matza in its waning moments before it expired.
I ate matzah every day of Pesach in accordance with this view, and at around 5:30 on the 8th day of Pesach, had a small meal of matza.