There's several ways characters can acquire Major Virtues, among them Mystery Initiation and experience in the Magic Realm. We've always ruled that a grog can only have Minor Virtues and Flaws, and should a Major one be acquired the grog becomes too important to remain a grog - and must become either a Companion or an NPC.
But we've just realized that a) any sufficiently warped grog starts getting Major Flaws and b) technically, Realms of Power: Magic has rules for grogs that acquire the Major Virtue Transformed by Magic. So it seems that the restriction only applies to starting Virtues and Flaws.
If the circumstances of the saga warrented a grog "acquiring" the equivilent of a major virtue, then I would certainly allow it. To my mind, the assignment of virtues and flaws really happens a character creation and some, especially social status virtues, change during play.
If a grog started out as a young squire it would be possible, over the course of the life of the magi, for that grog to become a knight and eventually even a wealthy landed noble (possibly as the covenant's "pet noble"). If the characters role remained that of a "supporting character" rather than a "central character in the stories" then there would be no real reason to consider him anything other than a grog, albeit a venerable grog in nominal control of a half-dozen manors.
Hmmm... I find it interesting that the comment on this in the text is not included on the chart, even though there is sufficient space.
Anyway, as others have pointed out before me, grogs may start with major virtues/flaws according to the rules. There are two groups of listings. For example, for flaws there are the "may not" group and the "should not" group. The wording is a little different but to the same effect with virtues. So, yes, grogs are allowed to have major virtues and flaws, but this is generally to be discouraged.