The "Bash" +10 is literally 10 extra damage on your shield bash (or weapon bash if subbing DRK.) E.G. if your shield bash usually hits for 17, it will hit for 27 with this earring. The interruption down is more situational than anything. If you're going to be tanking multiple enemies, or something with a high attack speed it can be useful, but if you can manage to time your cures between a monster's attacks this really doesn't do much.
Regarding some of the more recent comments. I look at this earring as potential for getting non-native pld spells up. Utsusemi: Ichi, Stoneskin, Phalanx, longer-cast barspeels like barsleep/ra. And though it's rare, even flash, ni, and stun get interrupted from time to time.
As usual, many people don't consider the possibility to macro this in. It's definitely not worth wearing full time, and cures can easily be timed in between attacks, but getting Utsusemi: Ichi up is really not that easy, even if you're very experienced with it lag can screw you over. Also it's useful when being attacked by multiple enemies, being able to cast spells while four mobs are hitting on you is a lifesaver. So this is really not that bad, combine it with Valor Breeches and that already makes 19% spell interruption rate down, which is a lot. If you still need more, there's other items you can get for PLD as well, like Willpower Torque or Resolute Belt.
The only thing about this is the price tag, because you can usually get other spell interruption rate down items for less. If you want to "finish" your PLD though, go for it, it's a very handy macro piece.
@Kujata.Omnys: PLD has native enhancing skill (210 @75), so when subbing RDM or WHM they will have the benefits of that higher skill towards interruption already for Stoneskin/Phalanx/Barspells/etc. Not that the extra % doesn't help, just clarifying.
This earring is rape if stacked with Fenian Ring, AFv2 gloves for shield bash. Also in mid-cast of Utsusemi: Ichi throw on this earring and AFv2 legs, can also help finish your cast if you didn't shield block.
"I look at this earring as potential for getting non-native pld spells up."
Um... your phalanx and stoneskin examples although they don't say PLD on the scroll with the correct sub, ie. RDM there is actually no difference in spell interuption rate due to skill as PLD have a native enhancing skill. Don't get me wrong your other examples are stalwart(no pun intended) and this earring does have usage in native pld spells also but just putting that out there that theres no difference in skill just because PLD can't learn it. Just puttin' that out there ^_^
