Harder dungeons would indeed be lovely, especially because midcore players tend to be less organized than raiders and thus may not have an 8 man static to run things with. The daily eso roulette could be the level 60 one, and the 2 new ones in a patch could be harder and offer unique gear or vanity. They would be nerfed by gear progress by the time they get demoted to the level 60 roulette.
On the other hand, any content but the top-tier stuff allows healers and tanks to dps very easily and very often once they are used to the encounters. In savage (and perhaps extremes) the extra dps is somewhat required, but elsewhere it's trivial to do. I think the question shouldn't be "Can I get away with not adding dps?", but rather "How can I help my party even more?". It's comparable to dps players not using their defensive skills, raise, Virus, mp/tp buffs or E4E with the excuses that those skills are not needed in non-top-tier content or that they are not directly related to their main role. Everyone has their own play style, but I think the ones that are more efficient and make the team stronger should be encouraged.
In the end the efficiency of a play style comes down to how the game is designed. Trinity in this game is not very strict, as evidenced by the fact that every role is able to put out enough damage and survive long enough when soloing. Having stricter class roles, perhaps with the addition of the fourth role of support, would make the leveling experience very different but would also force classes to focus on their own strengths to make the party stronger. Dps players and healers would be squishy, tanks couldn't self heal or do much dps, and healers would only be able to heal. This is not the case in FFXIV, in fact it's quite the opposite. DRG and SMN can tank situationally, healer dps is insanely good and tanks can push both healing and dps. Cross class abilities are a thing and mandatory for end game. These facts make me believe that hybrid play is is intended by design.