Dips without any RP reason are bad, yes. And if it's just single class feature you're after, it may be better to talk to your DM about switching one your class features for it.
Multiclassing is something that should always have a RP justification and it should be rare.
I guess I disagree strongly with a lot the guys said in the video...
Dips have rp reasons though...
Wanting to be a big angry wild shape is an rp reason. Not a complex one but a reason none the less.
I think the guys have the correct take here. Back up but how popular their content is. If you step back and look objectively you'll probably see that many of those who are against dips simply want to restrict the other players and have them as window dressing. It's a definite red flag.
I still disagree with their views and that's highly unlikely to ever change. I simply dislike their take on RP, which seems to be largely about player freedom.
I can safely say that I couldn't play in any of their groups, I prefer a different type of RP.
To me a player's desire will always be secondary to the setting if a conflict arises between the two.
So I guess we just have to agree to disagree.