Pika's Dair is so useful in quite a few situations. I think the best thing about it is that it stays out for a pretty good amount of time, and it's strong throughout the entire animation unlike Nair/Bair (although those 2 attacks do stay out longer).
Dair is a move I like to use mostly offstage to edgeguard someone if they're not in a good position to be tailspiked. For instance...
If a Fox is recovering and drops down really low to use his up-b straight upwards, then you can run off the stage and Dair. It has enough priority to go through his fire, and ends soon enough for you to double jump and up-b back onto the stage (except for on Yoshi's Story).
Also vs. opponents who like to jump a lot, doing a running full jump Dair can catch them a lot of times
It's good cuz it auto cancels when you full jump, so you don't have to worry about L canceling or anything. You can actually double jump before you land, although the timing for that can be a little tough.
Here's a few other useful times to use Dair against Marth =P
http://www.youtube.com/watch?v=aIJlDaFY7j8#t=7m46s