The Ermine's tail "raw" damage per second (or minute, or whatever) is better, but the +7/+10 attack from the B.K. off-hand affects not only the B.K.'s DPS, but your main hand's as well.. As well as your Weapon Skills.
Ermine's Tail is an evasion dagger and a soloing dagger, as far as I am concerned. The higher DPS is going to tempt some dancers into using it full time, but I'm not sure the loss of 7~10 attack is worth it in meripos or other situations.
DPS matters more than simply uninterupted melee rounds, however. It matters whenever you're hitting the mob everytime you can (i.e. if you're kiting a NM and not swinging everytime you could, low damage/low delay weapons lose out to high damage/high delay). Also, the faster your off-hand, the more frequent your attack rounds. Assuming a theorical DMG:99 main hand dagger, you'd want to use a theorical Dly:1 off-hand if you could, since the total delay of your round is average of both daggers, but your damage isn't averaged out.
Now, since we're not talking about fiction weapons, and the choices we have are limited, sometimes you'll want an off-hand for stats, sometimes for delay, sometimes for DPS... Ermine's main appeal, in my impression, is the stats. (evasion)
FFXI: Nyiah of Sylph -Retired- Docent of Valefor, Lianda of Kujata.
FFXIV: Cloe Delisle
Pugilist and Conjurer, leader of the 'A Mari Usque Ad Mare' Linkshell, Lindblum server.