So is job changing bad? Since it seems to revert the character's stats to lvl 1 (the bonus points from character creation also vanishes which can range from 6 points to 60 points worth of stats). Switching back to your previous job also does not retain levels (Thief lvl 13 switched to lvl 1 bard, switched back to thief and thief turned lvl 1).
depends on what level you and the rest of the party is. There's not much reason to swap around a level 1 character to a bunch of different classes, even if you qualify for them. Not unless you have a stockpile of spellbooks for them to learn through, which generally requires a lot of progression in the game already.
That said, doing the job-change tango can be very beneficial in the long run. As long as you continue qualifying for the classes you are changing to. As far as I could tell, permanent stat bonuses from items (such as the elixirs that drop from high level enemies sometimes and the barrels scattered across the entire game) are retained on class change. And when you class change, the level isn't retained, but you keep a notable fraction of your experience points and are issued a number of night tickets to help you re-level.
I made extensive use of job changing in my play through, taking my protagonist up to having mid level holy spells (level 5 spells in particular, because I saw raise dead and figure that would be good enough), then started having him be my party's summoner. While summoning was bugged for me, I kept him as a summoner for a while to try and grab a few more summons, then made him a mage (he couldn't be a sorcerer due to alignment of Pure Good). Meanwhile most of the rest of the party was leveling up just fine, in the mid teens range. The important part was that I noticed how easy it was to re-level my party members both from night tickets and from having a few higher level party members to hold down the front line and abusing spells to take out enemies.
So I started swapping most of my party through classes to learn various spells. By the end of the game, I've got a party with 5 people that have capped out spirit magic, capped out holy, capped out alchemy, and one person with the full summoning list (I created a character to be my dedicated summoner, then after completing the list of summons, had her swap through a few classes, dropping from level ~60something to ~28 by the end, but she went through mage and druid along the way to thief to get given a theif dagger and become an assassin). The person I class changed the least was Max, because fuck Max. I made him an alchemist long enough to max out alchemy, then found my first thief dagger and changed him back to become an assassin and left him like that since. He's mostly got alchemy for me to fall back on his spell slots for utility, because his combat actions are generally 'attack twice, kill two monsters either because obscene damage or instant kill procs'.
Yes, I have two assassins in my party at end game. Why wouldn't I? Thief dagger is an item drop and using it ignores the alignment requirement for assassin. If I could just get a second excalibur to drop, then I'd be golden, I spent the one I found as a random drop in the forest of 'go wander around the no warp teleport maze of undead and demons' turning my protagnist into a lord as his final class change (after running through hunter to update his summon list, which is still incomplete, but I'm keeping him as a lord because I have no way to get him back to pure good to go back to lord the normal way).
TLDR: class changing can let you do some real shenanigans, but it works best when you're willing to grind.
Edit:
The Harp of Sleep can be gained by either making a fresh new bard at the party management place, or by smacking around enemies in the early level areas (such as the forest north of sapphire city)