To explain in a bit more detail: Currently, there is a HUD element for currency. By default, it displays gil, but clicking on it switches it to display GC seals instead. When more currencies were added, they were not added to this HUD display element, which does make sense. Gil and GC seals can be farmed for or obtained individually, which is untrue for other types of currency.

While it would be nice to be able to see Tomestones and Allied Seals in the currency HUD element, it makes some sense that they not be included there. Tomestones are a multi-dimensional and transient form of currency - not only do more types of tomestones get added and old ones removed, the vast majority of the time, when you gain tomestones, you gain two types at once. If they had been added, it could be confusing when a new type was added or an old type was removed. Right now, would there be a Mythology count despite it being 0 for 99% of the players? Would there be a Philosophy count still even though those have been unobtainable for quite a long time? What if someone had philo displayed when they logged off way back when, and they log in now? What is displayed for them? These ambiguities make it make total sense that Tomestones were not added to the HUD display element.

Allied Seals make more sense to be added, but again, you cannot obtain them without also obtaining tomestones. It is also unclear if SE is ever going to add a new type of allied seal from hunts in expansion packs. So, again, I can see them not wanting to add them to the HUD display.

But, that brings me to MGP. MGP will exist into the future, and there do not appear to be plans for more forms of MGP. MGP can only be farmed by itself. When you are gathering MGP, you only care about your MGP total - gil is largely irrelevant and GC Seals are completely irrelevant. Having to go into a specialized menu to see the total is a bit annoying, especially if you're in the middle of a minigame or triple triad match where you can't bring up the menu at all until you're done. I don't see a reason not to include them in the HUD element along with GC Seals and gil.