It's only small def debuff, easily countered by invincible and/or tough hide.
Think its there from when they used to make you immovable, which makes sense really 
It pretty much explains itself in the description, which, IIRC, says "Your Unyielding stance makes you easier to hit, slightly reducing your defence" or similar. It's about -5% def I think. Fair enough for a power that gives you outstanding all round mez/KB resistance. Gotta be a vast improvement on being rooted to the spot whilst using Unyielding, which I understand used to be the case (shudder)
Just looking at the powers in the invulnerability set and notice that unyielding has res and -defence. The -def seems really odd for a tank set.
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Fiend Space: 211464