For those who don't know, "session zero" is a meeting (generally taking about one session, ergo the name) between the players and DM, discussing the upcoming campaign, character creation. It's very helpful for making sure that everyone is on the same page, which is very helpful for maintaining tone. It (generally) prevents everyone from making serious characters with their tragic backstories and then one guy who makes a gnome who thinks he's a giant called "Thicc E. McHottness".

That's all fine and good, but there are times when I find that session zero impedes the fun. The two biggest examples are one-shots and new players. They're best served by 2-3 sentences of what the adventure will be, followed by character making as fast as possible, so you can actually play the game. If you believe that constitutes a "session zero", we're only disagreeing on definitions and we'll move on.

One-shots (a single session with a full adventure packed in it)
This isn't likely a controversial statement. Doubling the run time of your adventure will absolutely murder the pacing and mean half of your play time isn't spent playing.

New players
This one will probably get an angry mob after me (oddly easy on the internet), but hear me out.
The average session zero is THREE HOURS. Your poor friend has never played D&D, probably doesn't know the rules, and the best way to kill the enthusiasm is to expound on rules, team building, house rules, etc. etc. and not actually play the game. People want to play the game and if you give them a lecture the length of Lord of the Rings before you let them play, I don't blame anyone for ducking out.

Children
This is much the same as new players. Just let them play the game. Worry about the minutia later. On a side note, just always assume these games will end up comedic. Use age-appropriate adventures and everything will be fine.

It's a game. If someone doesn't know they'll love it yet, just let them play.