imo, this serves the same purpose as Bushinomimi does for Triumph earring. the additional stats might not make much of a difference, but the main stat basically replaces a fairly expensive piece for free. I'd wear it over Merman's if I had it. the 1 atk difference is negligible and no -EVA.
I got one of these on my 8th ??? Earring from Lebros Cavern Excavation duty. I don't know if where you get the earring effects what it can be appraised as, but just incase. ^^;
The cost varies by server, but a Merman's Earring gives 6 attack, -2% magic damage taken, and -4 evasion, 13 levels later than this. I can't really see any reason to use a merman's over this, even if it didn't have the +10 additional attack in assault. +1 attack, 2% less magic dmg taken, and 4 less evasion is not worth the price tag.