nah...

The answer was because the adamant armor gives the wearer immunity to every statuses (except berserk), which includes Float... Immunity to Float... woah... Yes, try it

So if you go to the Land of Summoned Monsters or the Sylph Cave with this armor, you either get a lot of damage, or take off the armor and cast Float.

Another good answer would be that it enters in conflict with the Cursed Ring, which gives Elemental Absorption to the selected elements. The Adamant Armor give Elemental Immunity to them. So lets say you are wearing the Dragoon Shield (Fire, Ice and Lightning bits) and the Cursed Ring (Absorb bit)... You are going to absorb Fire, Ice and Lightning. Wearing the Adamant Armor with them sets the Immune bit, so you end up getting no damage for Fire, Ice and Lightning, instead of absorbing the damage...

*sigh* I really hope these two little pieces of info about the Adamant Armor was interresting (any feedback? )

I'll ask another question instead...
The little FloatEye (near Baron... yeah, the blue winged eyeball) has two kinds of attack. What are they?