Even if it's every 15 min, you earn only 1000 MGP (more or less depends of the GATE)
It's still so small to compensate the fact that you need to "camp them" to be able to "farm" them... playing TT against the lazy garlean guard is still better reward for low difficulty...
It's the same for the chocobo race, if the reward were better, people would grind them a bit more, right now, it's like 3 min for 170 MGP at low level (and it's a looooong way to be high level) Playing the new minigame (botanist precisely) is much easier and faster too. (i can win at least 160MGP every minute or so)

TL;DR : The MGP reward are too low for "casual playing", you must farm them 24/7 to earn 1M MGP... (and it still not enough)