It has more lag when you do it in the air and have to land after it, I think. Either way it's not a safe option unless you know you're going to mix them up enough to hit them.
I like to use DC in general for a few specific things, mostly spacing and mind games. A lot of the time if they're being forced into shield, I'll do an empty short hop at IDC distance away, and this makes them think they have an opening right as I land, so they let go of shield. Then, I just IDC to them and hit them, and this works on almost anyone if you can get them scared and get into their head.
I'll do another similar thing with an empty DC, maybe after something like a d-throw, or just at some point when they're trying to escape pressure and are likely to make a bad choice. I'll DC and have them think I'm coming towards them, but in reality I'll space to land in order to not be in any sort of danger and be right at the end of the opponent's range, and then just take a step or 2 away and f-smash. You can catch soooooo many people out of something like a dash attack like this, even without the empty DC. I know it's not IDC, but it's still a really good use of the down-b if you can do it effectively.
But yeah, the main uses of IDC for me are as a fake-out with an empty short-hop, and comboing out of an up-throw at certain percents, although at around 100% of fast-fallers you can combo a fully charged DC instead of an IDC. It had more limited uses now since you can't combo d-throw IDC as well.