Falco can RAR a bair pretty effectively - a bit more range than nair and the landing lag l-cancelled is 3 more frames (7 vs 10 if the same as Melee), so it's a mixup on shields (harder to shield-grab at least - they probably need to wavedash OoS to try to grab you) and if it connects, it's a better KO move at high percents compared to nair. This was much more true in Brawl than in PM, since in Brawl his fair had horrible landing lag and his nair has low range, but it still works well in P:M.
I'd also imagine Mario gets some mileage out of this for trying to go in (probably a lot better than fair on shields), the same would seem to be true for Samus (also for KOing), Kirby (KO or approach), etc... really anyone with a bair that has more utility than their fair, either because you think they'll shield it but want to set up some close-quarters-combat or because it's a better KO move.
*realizes just now that you main Ike*
You should definitely try it - Ike's bair is pretty awesome to RAR with - high knockback and much faster than fair and even faster than nair I believe. Plus at long range I'm pretty sure it's rather safe on shields with proper timing and L-cancelling.
So if you want to rush in, but don't want to leave yourself as vulnerable with fair or nair, try to RAR a bair. That's the main situation it's good for.
It can also be good in edgeguards - running to the edge and quickly turning and jumping, you'll still keep your momentum while facing the opposite direction - so if you have a good bair to edgeguard with (I dunno about Ike, as his fair is a pretty edgeguard, but Captain Falcon is a perfect example) it can be a great way to get farther offstage without using a second jump (like moonwalking offstage almost).