Fast and smooth is great in optional end game content you're trying to farm with your friends. In a leveling experience, things going too fast and too smooth absolutely is a bad thing.
Humans don't learn well from simply observing once they're past 4 or 5 years of age, especially when they're not really certain what they're observing. We learn well from doing and from making mistakes.
If things are dying before the player has a chance to get off more than their opening attack, they're not learning how to use the rest of their toolkit. Watching someone slaughter things as a Dragoon doesn't do anything to teach them how to play Thaumaturge (or Black Mage if they're past level 30).
They won't learn game mechanics if bosses are dying or getting pushed into phase transitions before those mechanics can occur. Assuming that those mechanics even exist in leveling content, of course. It's absolutely hysterical watching guides for SB dungeons saying "you should know this mechanic by now" when that mechanic didn't exist in leveling content. It had only existed in optional end game raids or EX trials that not everyone player does (and which a new player almost certainly hasn't done).
Fast and smooth also makes for a boring experience. You don't care, you were only there for your tomestones and you want to get out as fast as possible. What about the player who's trying to decide if they like the game? How are they going to gain interest in a game where everything dies without thought so they never feel challenged?
What are they going to do when they finally catch up to other players, the content suddenly gets hard and they don't have a clue what they're doing so other players start hating on them?
Just maybe it's better for that leveling experience not to be so fast and so smooth so players can actually learn something and they're better prepared for the more difficult content that's ahead of them.