THM screams RDM in every way possible, yet I have never seen anyone say anything but "turn THM into WHM or BLM"
THM has the HP/MP regen abilities like RDM did, it has all the good enfeebling spells that RDM did, and it has the soloability RDM did. If you haven't seen the now old videos of THMs soloing Dodore and Hautpox (it's not so hard given that Emulate is ridiculous), you might start to see just how much like RDM THM is.
I think I can provide an insight on that. Red Mage, in a way, was a failure of a class from the very beginning. It posed a threat to the WHM because of Refresh/Convert, making RDM the more ideal Healer/Buffer/Debuffer. At the same time it snuffed BLMs role in the party as a DD/Debuffer, since RDM could debuff better than a BLM and still provide some DD using both Magic(Mostly DoT) and Melee(yes, I said melee, I don't care how many people say a RDM shouldn't melee, I did it from 1-75 and still was able to main heal, debuff, buff, etc...). BLM could obviously DD beter than RDM, but because BLM's hate would spike, and because they were so squishy, a lot of parties passed them up for a melee job, forcing them to form "burn parties" (5 BLM + 1 RDM typically) to level quickly. The eventual result (before the release of Scholar) was the exile of BLMs and WHMs from XP grinding PTs in favor of an MP efficient healer/debuffer/buffer that never needed to rest MP.
This time around I think SE was concerned about releasing 3 starter mage classes, fearing a similar thing could happen later on. Instead, they made THM, a Heaven-and-**** style job that uses raw Umbral(or Dark) magic and Astral (or Light) magic. Combined with abilities fitting the theme (Bio/Dia, Sacrifice(Healing magic, but at the cost of your own HP making them useless when your HP is low), and an array of Absorbing spells) they made a class with DD/Debuffing/Healing potential. Initiation warrants mentioning as it allows THM to transfer up to 4 Buffs from the THM to another player. (For example the THM could draw ACC/ATT/DEF/EVA from a monster and give the absorbed stats to the Tank.) They made CON an elementalist. Giving access to the various element specific spells and Ancient Magic as well as the popular party buffs Protect/Shell/Stoneskin. Along with the Cure spells, CON is more of an ideal healer (assuming you couldn't cross abilities) and damage dealer, with its ability to abuse a specific elemental weakness of a monster. They further divided RDM-esque abilites amongst the two mage classes by giving CON Fastcast and Chainspell. Now with the cross-class skill swapping THM and CON can be nearly duplicate in any party as far as dealing damage, debuffing, buffing, healing. This is the root of the new problem though. By trying to avoid the Red Mage syndrome from FFXI, they created a new "virus" so to speak.
What we are left with is a BLM that isn't a BLM, and a WHM that isn't a WHM. Instead we have two RDMs with a different type of AoE, and neither one is really as good as it should be. Both THM and CON are lacking in the "impressive" damage department with their high rank abilities, and effectiveness of their debuffs, (they don't ever last long), on the other hand, buffs have a short duration, forcing a constant recast of these abilities. CON ability to remove status effects is almost completely pointless because fights don't last long, and neither do the status effects. I have heard many times from people that they don't bother to try to remove effects because by the time they cast the Blinda or whatever, the effect has already worn off.
One other little tid bit that seems to make the two near carbon-copy mage classes is the lack of differentiation between single-target and AoE spells. Single Target spells in FFXI were mor potent then they're AoE counter parts in most cases, and the AoEs cost more MP. Currently ST and AoE cost an equal amount of MP and the effects are not recuded regardless of the number of targets (as far as heals/Siphon MP goes at least).
To wrap this all up, SE feared a RDM class pushing aside any BLM or WHM class, and so divied it up between the two and ended up balancing the two jobs -too well-.
Given the current state of the two, it will take some shifting of abilities, and maybe some more strict MP costs, and perhaps a differentiation between ST and AoE spells, to give the two classes a distinguishing role in the party.
For now however, if I needed a healer in a party all I need to say is "I need a mage." Although I guess it is still slightly better than saying "I need a RDM." and having the BLMs and WHMs Shoop da Whoop the forums with "Why can't I get a party?" threads.
Well I am done firin' meh lazor. I am sure some will agree and disagree, but I hope this at least brings a little light to the topic, and inspires some good discussions, even if I don't please everyone with my opinion.