Good choice even if you do have Medi. Her damage is comparable, but having Holy gives her an out to Evade tanks, since she won't waste a lot of AP on an LB when it isn't needed she can cast for longer, and her white mage subjob means she can be a good healer when you don't have the ability to have a dedicated healing unit and lack Ayaka. Her TMR is also a great thing to have on magic units.
Long after you have benched Mont, Rairyuu, and Y'shtola, Vistralle will still be useful. Especially for steal heart and steal time in raids. Also, his steal skill is useful for farming mats needed in crafting. Oh, and I always bring him along for daily cactuar and gil maps.
I dunno, I use him a lot. Always save my day with steal heart and his TMR its pretty useful to add up some extra dark eater and eva (15 eva + dark eater). Good for some eva teams if you don't have all the URs maxed (Vinera, Kitone...)
Rating explanation section is too short. Seems like MR units aren't many used in battles.