Viper has no defensive because of... dragoon

Everyone seems to have forgotten that drg has never had a true defensive skill - it has an evade, something that's actively detrimental in the situations you'd use a defensive for (towers, stacks, and raid-wides)

Because drg exists, and doesn't have a defensive, all forms of unavoidable damage have to be balanced around it. You can't have a mechanic that requires a dps defensive(beyond feint), or that mechanic is un-resolvable with a drg in party

Since they were already forced to make content balanced around drg not having a defensive, there was no mechanical reason viper needed one. So it becomes a pure flavor choice... and having this one-off button actually goes against the "flow state rhythm of fighting" that viper seems to be designed for.