its useful in rare situations, but i see where your coming from, especially when you have the ability to just DJC a headbutt. This, though is a situation where you need the up tilt. It is your move that sends people up with the most priority. Almost nothing can stop the up tilt, not even link's pogo if you time it right. That means if there is a character like Cfalcon, who comes from above and does a down a spike, you can up tilt, and follow that up with a headbutt. You use it to continue aerial combos when your opponent is high enough to retaliate with a Dair, cause as soon as he comes down to make the hit, you wack him and he goes straight up. Additionally, it sends people higher than the headbutt when htey are at low percentages, meaning if a jiggly comes around, you can use it when she tries to get above you and it sends her high enough to get more breathing room. Mainly, use it to continue aerial combos, or if you want to create some breathing space between you and an opponent, because sometimes the headbutt wont cut it as you'll simply deal damage, but they get a chance to counterattack.