Fun is a very subjective issue. This game really only allows you to play within the confines of community acceptance. Some would say to have fun they would play it their way and use less acceptable job combinations. However, can they really have fun if they sit around for days with their flag up looking for an invite?
There is generally reasons that specific job combinations are acceptable. Some are good and some are pretty bad. Most often job combos are acceptable because they've been tried and proven to be the optimal usage of job and subjob. There are many mage/melee and melee/mage combos that are just not very viable because they offer nothing in combination except for situational use. (i.e. SMN/NIN is worthless in most situations, but with SMN/NIN and BLM/NIN my wife and I duo Mission 2-3 for people often)
However, the other big reason certain combos are acceptable is because people aren't open minded enough to see the actual benefits of a combination vs the most common combinations.
WAR/DRG is actually a great combo from 20-30 if you do not plan to tank. /DRG is great for a lot of 2 handed weapon users period. If you aren't planning to tank there's little need to /WAR or /MNK and almost no reason to /NIN (especially pre-50). However, people will very much laugh at you for /DRG.
Another example is NIN/BLM 51+. NIN is a potentialliy incredible DD with the ability to eat hits when the pull hate, but no NIN is allowed to try this.
I think people should be somewhat open to new combinations, but it also seems dangerous to suggest to inexperienced players that there are any job combos outside the box. Having PT'd with a good number of RDM/THFs and RDM/NINs (using swords or daggers instead of clubs) I definitely see the danger.
So what is fun? Is playing an acceptable combo fun while getting invites, or is playing a slightly off combination (for better or worse) fun while getting less invites?
Should we encourage people to try other viable subs or is it too dangerous due to the misunderstandings and inevitable bad combos?