In a game where you have more than one class/job, designing things to exclude some of the jobs/classes is a bad idea.
No, it's not a bad idea, as long as there is content for every job and the amount of which is balanced.

There will be no thought process to forming a party, and nothing interesting about being in one, if you can randomly invite people regardless of job and beat the content. There has to be pros and cons to having each class. Otherwise the game becomes completely uninteresting because none of your choices matter.