R’ Moshe Meir Weiss writes that Rav Shach who lived to 102 attributed his longevity to always bentching with a bentcher.    Another reason he suggests to always bentch with a bentcher is so “one’s eyes should also have a part in this wonderful mitzvah.  If one bentches by heart, the eyes aren’t taking an active role, and thus miss out on the holiness acquired from the lofty activity.”