You got to keep in mind, you're not actually meant to have EVERY spell, in the typical offline games. This is why so many of the games gave BLU melee potential, and allow such a jack of all trades stat build.
They didnt know which spells u would have, and or not have. so BLU needed to adapt to which ever spells the player happened to have, and still be usable. (Strago went full mage, because you had access to spells right from the start)
Also, most jobs had primarily offensive options, so its still not fair to say something is "DPS with X flavor added". Not to say I disagree with your final assessment of what a BLU was to the developers. But those FFs functioned so much more differently, trying to make comparisons in that way is also kind of faulty. Currently, the three versions of BLU we have in multiplayer games have all been extremely good interpretations of classic BLU. (Even if I absolutely hate 14s BLU, and even more so its "Limited" title.)
Theres also the fact that if SE were to ever make a full "Cloth caster tank", they would either have to make a job never before made, or pick from the jobs which come "the closest" to this idea. Similar if they were to ever make a "Melee Healer", there's only so many classic FF jobs to fit the bill, w/o starting to just feel like a reskinned WHM.
I find the current "DPS main, sub tank/healer" style to BLU fine, as well as the "Multi-role" desire that some people have for it.
lastly... about SMN as a DPS, ironically enough... SMN has been my main healer is so many of the FFs, if not most of them lol. Even in FF4, Rydia was more useful spamming asura for me, than having Rosa spam heals.
Kain was the only one who could survive the last boss, so he was a mix between a DPS DRG and reviving the SMN. The SMN revived everyone else with asura 33% of the time.
If that fails, DRG jumps, to survive the next raid wide attack, while the SMN keeps trying to party rez. Rosa spammed Holy. And Cecil spammed his holy sword.
I pretty much used them in reverse of their normal roles. The PLD and WHM were my heavy hitter DPS, and the DRG and SMN were my tank/healers. lol...
(This is also probably because i always run from random encounters, and i was severely under leveled, so it was the ONLY method that worked lol)