Well, 1.5 introduced something that is guaranteed to eat up some prismatic shards once you get to/unlock it.
If I don't need them for that, well, I love putting 4 in each Junimo Hut so that my Junimos go prismatic. That's always fun.
If I don't need them for anything like the above, and if I don't really need the money... well, they can be saved for gifting, as everyone loves them except for Haley. Edit to add: and I have plenty of rabbit feet to give to her when I get asked to give out 50 loved gifts in a week.