When the Duty Roulettes were introduced in
ARR Patch 2.1, the requirements for what was then called "Duty Roulette: High Level", were just unlocking two duties that belong to the roulette. At the time, Pharos Sirius had a reputation for being (a) difficult and (b) long, so many people avoided unlocking it at all so that they wouldn't have to do it in roulette. To combat this,
ARR Patch 2.2 changed the requirement for DR:High Level to be "completion of all duties" instead of just "unlock two or more duties". This forced the people who were avoiding Pharos Sirius to unlock it or give up the roulette.
Since then, DR: High Level has become DR:50 then DR:50/60 and now DR:AllOldCapLevels and the requirement re-relaxed to just unlocking of duties because it spans multiple cap levels and one might want to use it on a character that is, e.g., Lv60 and therefore unable to access Lv70 duties.
If given an option to exclude any duty from a roulette for any reason, some people will use it to avoid doing a duty ever. If anything, the current roulette requirements are already too lax--people can unlock just MSQ-required dungeons and never have to run any others in their roulette. The roulette should require unlocking all duties up to your current job's level so that people can't tailor their roulettes to a specific duty set.
At any rate, adding an option to exclude unlocked but incomplete duties from a roulette so that people can save certain duties for certain friends is not worth the implementation time when you consider the abuse potential and that people who want to save certain duties for certain friends can already do so by simply not running their roulette until they've run the certain duty with the certain friends.