Oh, you should definitely do the whole dungeon sometime for fun if nothing else! Groups wanting to do the dungeon quick for random or pledge usually skip not only those, but any mobs that can be as well, as you’ve probably experienced. I don’t believe the first three secret bosses drop anything special, although there will be a chest in one of their areas, but they give a buff that can be useful if doing the hard modes. The first three secret bosses are easy but the fourth, I don’t think I’ve defeated yet, and you are limited to five or six attempts.

Yes, done it a few times. Still haven't got the momento that allows you to see the secret ice chests though

the side bosses do not drop weapons, even the 4th secret boss, so anyone who is farming weapons just skips those normally

its great though if you havent run the dungeon much and missing a lot of the gear though

You don't really get anything for beating the final secret boss hard mode except 1 extra chest. Not even an achievement.