Well my reasoning for being able to have a different wpn for BLU is that DRK keeps being brought up as a 2nd job for GLA quite often. It's certainly not using a 1 handed sword. It will most likely use a 2 handed sword which PLD most likely won't be able to use.
Maybe Fencer (maybe Duelist instead) can use both Degens and Scimitars but RDM can only use Degens and BLU can only use Scimitars. From my past experiences both RDM and BLU are jack-of-all-trades and particularly in FFXI both can fulfill all roles.
I've Tanked(including non /NIN)/DPSed/Healed/Supported on both RDM and BLU at Lv 75 cap, and sometimes I even mixed the roles and performed multiple ones at lesser values at the same time depending on the party setup and the fight. However subjobs really helped RDM and BLU perform other roles, basically with a jack-of-all-trades you were a master of none but a subjob greatly closed that gap. In theory from my perspective RDM is 1/3 WHM 1/3 BLM and 1/3 WAR from the original concepts.
BLU is even more versatile than that.
My issue with SCH is that my only experience with the job was in FFXI, which was 1/2 WHM and 1/2 BLM (which is what the majority of people considered RDM in FFXI to be). In FFXIV the way SCH is setup is cool I just don't feel like it deserves that name at all. The AF and lore matches the name perfectly, but for the me the actions don't. This is why I feel SCH was thrown under the bus. Sure SCH can do dmg, but so can WHM and in most scenarios where you would want a healer to also do dmg (WP SR or for a quick burst) I would honestly take a WHM over a SCH. SCH would be better DPS for a single-target/longer fight (also AOE if longer) than WHM, but in EG you just don't see healers in the DPS role unless the party far outclasses the content in skill/gear. For example I have ran AK with 4 DPS roles and solo tanked/solo healed with Titan-Egi as SMN w/o any wipes, but that just isn't going to be viable in EG.
Also, while I like SMN (it's my main), it feels MORE like a Warlock from WoW than a FF style SMN. SMN is also a cool setup but like SCH (and yet again the AF and lore matches the name) I don't feel like it deserves that name. I don't want RDM and BLU to end up like these jobs, where the name doesn't fit the actions/play style (which I consider being thrown under the bus).