Grandparental relationships

Bava Basra 128a. The gemara discusses the testimony (being a witness) for a close relative. Mar Bar Rav Ashi notes that a grandchild may testify for a grandfather. The gemara continues that we do not pasken that way. The Rashbam and Rashi (as brought by Tosfos) defines this as not being the halacha for all descendents. Tosfos though brings the R"i that disagrees when many generations intercede. It is possible that this is a question of why the halacha is not like this. Is grandparenthood about loyalty or personal relationship.