The Vis gained instead of xp while in the Magic Realm is special in that it can be used to "transform" a character who does not have Might, where as any other Vis found in the Magic Realm or brought there by the character cannot. This is in general far more powerful than the effects that can normally be gained from Vis, so while it can be used for other things saving it to power transformations is normally a better choice.
Note that it is possible to gain XP while in the Magic Realm, but only by "comprehending the Twilight Void". This is a special case since success adds XP while failure loses it from an Art or Ability. If it is a gain, the person actually has the choice of adding the XP to the Art/Ability or converting it to Vis.
It can also be abused by someone with Enigmatic Wisdom and a high Boundary/Vestige Travel Score (Perception and an appropriate Ability, Second Sight or Magic Sensitivity, picking areas associated with highest Arts) who also has a high score in an Ability that provides Familiarity (Magic Lore, Enigmatic Wisdom, Area Lore [Magic Realm]). Characters who can consistently generate a high initial Travel Total (15+) can perform 1+ per day on average. Though having a high Twilight Void Travel Score (Stamina and Concentration, plus sticking to the area of your highest Art) is an important safety feature if trying this.
Trying to abuse Comprehend the Twilight Void is a sure path to Warping Points and Final Twilight for a being without Might however. For those with a low Might and high Enigmatic Wisdom, they can easily generate 30+ simple dice of XP/Vis (and points of Essential Traits) in a season. Higher totals scale this almost infinitely. The points of Essential Traits can actually be more valuable than the XP/Vis since they can give effective Attributes up to 10 in limited areas and provide limited immunity to magical effects.
I would never recommend trying this though, since botches are a very real danger and when you are dealing with so many rolls rapidly become a very real probability. Even "Cautious with Ability" Virtues would not help much, since you would need three of them in Abilities not normally taken with (Second Sight/Magic Sensitivity, Concentration, and Enigmatic Wisdom).
EDIT: Note that all of the above is without Magic Realm Magic. If you have it and appropriate spells then much of the danger on the two Travel Scores are removed. However you still need a high Intelligence and Enigmatic Wisdom to pull it off.