If you're a completionist, it's definitely worth defeating the Unusual Hilichurl at least 50 times for the achievements and name card. The extra adventure rank XP is also great to grind for, but most people probably aren't prepared to defeat 10 Unusual Hilichurls a day, especially because you'll need to enter co-op to do so. If you're not huge on achievement hunting, then you probably won't have to worry about specifically hunting the Unusual Hilichurl, but it can still be a lot of fun to find him.