Missing verses are always interesting, and Mark has its fair share (7.16; 9.44, 46; 11.26; 15.28; 16.9-20). These are passages that were printed, with verse numbers assigned, in Stephanus 1551 but are no longer generally regarded as part of the original text of Mark and not printed as such in recent critical editions.
But what about Mark 15.28? Is rehabilitation possible? Or is this verse destined for life in the margins?
Evidence in support of this reading includes Eusebius (certainly was included in Eusebius' canon lists noting the parallel in Luke 277—this could reflect an earlier tradition). Other early evidence is in the Old Latin. So it was clearly in existence well before the fourth century (the date of our earliest manuscripts which lack the verse). It is also obviously no simple harmonisation to Luke 22.37 (since the introductory formula is quite different). It is clearly congruent with Markan style and forms a striking inclusio with the initial citation of Isaiah in Mark 1.2-3.
So against it is only a good selection of the best early manuscripts of Mark, the earliest witnesses in Syriac and Coptic, and the small transcriptional matter that its inclusion is far more easily explained than its omission.