There's activism happening around 4/20. My dispensary is getting folks to post their experiences with medical use, and getting them to sign a petition and giving out stickers promoting my state's recreational bill. Plus sometimes it's fun to have fun. Having a single day (and time) to make it a cultural event helps with activism by removing the stigma against it.
I don't celebrate 4/20 because I use it medicinally, but I'm glad it's a thing if only because it makes everybody forget Hitler's birthday.