In all honesty I feel all jobs specialized in only 1 role should be at minimum 10% more effective than a Hybrid Role able to switch roles mid-combat. At the same time there are already classes/jobs that aren't equal in pure DPS, but classes/jobs that have to ramp DPS are the highest (SMN and MNK) because they start off fairly slow. It only make sense that a class/job that starts off slower will be able to surpass other DPS after a certain point of time otherwise you wouldn't want to bring class/job for lack of burst (start of fight) w/o any benefit at the other end.
When it comes to the player choosing a class for himself/herself, sure a Hybrid Role may appeal to someone more than leveling 2 separate classes to perform 2 different roles (however I assume most people will still choose the single job that fits their ideal play style in mind over that convenience, as it will still be very viable in use), but keep in mind in the future all classes will eventually be able to perform multiple roles not on the same job but on the same class via a job for each role. So in essence you only level 1 class and whether that class has 1 Hybrid Role or 2-3 Specific Roles you still can perform different roles with 1 class.
Most likely. As for classes that have separate roles like ACN you have to carry a set of gear for both SMN and SCH if you want to play both. On my SMN I do more than just DPS, sometimes I support tank or solo tank (for fun) with Titan-Egi. Sometimes I just throw out some support heals if I feel it's needed. But I choose to wear i70 crafted jewelry with 9 INT, 9 MND, 9 VIT, 6 PIE over i90 INT slots. All Egi's have increased HP (they die less often and Sustain is more effective in keeping them alive). The character has more HP (great for PVE and PVP). I heal 450-470 with SMN cures (works pretty well with Crit Rate too). PIE affects both MP pool (also increases MP gained from Aetherflow) as well as MP regen (and both are very important for SMN sustaining dmg in longer fights while saving Aethers for burst mechanics).
What does SMN lost from -20 INT if compared to full i90 jewelry? About 5% dmg. Considering SMN is the #1/#2 DPS in most situations losing 5% dmg is still going to leave SMN at #1/#2 and even if it did slip down to #2/#3 then so what, but also at the same time if the fight is super long then SMN will actually gain dmg from a boost in PIE which also helps prepare for burst mechanics (IMO is far more important to deal with than pure DPS).
Pretty much. This is going to be the #1 issue for both Hybrid Roles and multiple job roles for any class, but it's still not too difficult to achieve and can be done on classes already released as Yoshi P stated mutliple times with the example of DRK job for the GLA class. But in comparison to the gear it can pose an issue to players not wanting to meld gear with attributes just to be super effective at multiple roles. SE needs to release JSE (job specific equipment) AND new specific role sets of gear (IE Fending for Tank, Healing for Healer, Caster for Mage) with multiple stats that the job will need, much like how healing sets have VIT, MND, and PIE (which replaces 1 secondary stat) instead of just 2 attributes.
The trade off for Hybrid Roles can be the loss of Secondary Stats for the benefit of Primary Attributes (which will benefit them overall far more). Hitting an ACC cap may become an issue though if they need 4 attributes unless those jobs do not require as much ACC like how DoM classes/jobs do not require as much ACC as DoW classes/jobs.