@ Habiki while some of the "spells" a Bluemage possess, I think the gameplay would be quite different. Saying a spell is used the same and not needed is like say why have redmage, dark night, and bluemages when a BLM can do it better. Just as all 3 of those jobs plays differently, just so would a "Psioncist"(yes the name is crap).
If this job is built with the same sorta of structure as a dancer, that is TP use, distinct categories, and several levels of power(steps) excepts as a more damage dealer or tank/control job, then I can see a place for this job.
While the list I provided was a bit large, SquareEnix could conceivably, limit the power to a few but power them up. As an example, "force push" being both straight damage and knowckback, could through TP cost, go from a relative mild attack and knock back effect to "step 5" a powerful damage and knock back effect. So to can this method be applied to the remaining powers.
Let's look at a Bluemage vs Psioncist comparison. Haste. A Bluemage casts refueling and receives a set amount if haste. A psionicist using "speed" could go from haste -1 > haste +3(I'm using general terms what the actual numbers is ?). Let look at defense. Cocoon gives what +100 defense or so? Imagine a Psioncist, which uses several tiers of "Ironskin" to go from +25 > +150 defense?
With various TP gaining abilities, a psionicist will be constantly maintain several levels of power and aggressive moves, rather than simply casting several head butts or solo skill chain.
I can envision several styles of play depending on abilities. While I originally, thought of this job as a ranged/dancer job, a tank could easily be accomplished through uses of tank abilities(Ironskin) and mind control(ie enmity moves such as several tries of "provoke").