I find Sword Oath to not be that great of a skill. In fact, the effects of it are quite insignificant (+50 to auto-attack potency, in a class where damage is low to begin with), that it's really no different than running without an Oath skill turned on.

Shield Oath however is pretty significant. The 20% damage reduction, and 20% damage dealt reduction is quite noticeable, but it's able to keep up a job of generating aggro (maybe a bit more) than with Sword Oath.

As I understand it, Warriors have Defiance - so it would really be the same as turning it off, to not get the 20% damage dealt reduction penalty.

One of the Warriors I do Titan HM with regularly equips DPS gear (ie. Hetairos Gear) just for Titan HM. Warriors will also be buffed as of 2.1, when Crystal Tower would also be released, so it might be better to wait until then and see whether the changes impact the presence of Warriors in parties.