Papalymo is not fond of you calling him "Potato" considering his partner has the intelligence of one!
That said, I was randomly musing about what characters could make use of Magic Swords, particularly the 6* ones, and here's my useless analysis! Note I haven't looked at RD to heavily see what skillsets get open, nor what characters get access to Swords.
First off, Tyro and Onion Knight don't count, as usual, getting that out of the way!
Next off, Roaming Warriors don't count; yes, technically any Sword user can use Raines' RW and spam his moves with a magic sword, but that's clearly a highly technical scenario, and not factoring that in, lest every swordsmen would be on this list.
So moving on, let's look at the characters who make blatant use of it. These are characters who are competent spellcasters. These characters have Black 5 (or Summon 5, though I don't think anyone on this list has Summon 5 without Black 5) and an expected Magic Stat. As a result, I won't be mentioning reasons, since they're all the same here:
Maria
Emperor
Desch
Golbez
Ex-death
Terra
Gogo-6
Ashe
Vayne
Serah
Raines
Jihl Nabaat
As an addendum, I can see Scott being added in the next event to the above list with a Desch-like Build. Hard to say with him, but considering FF2's Wild Rose increased Intellect to 99 and he joins with level 5 Fire/Blizzard/Lightning, I could definitely see a hybrid build on him.
Straight forward; the above characters are either Primarily Mages, or Hybrids and all of them can use swords. Moving on!
The next batch are those Darkness 5 characters who are not primary Mages, but given how strong the MM + DZ combo is, giving them competent magic weapon can in fact come in handy in niche situations and they can work as Makeshift mages. This does not take into account being obsoleted in that role in the same realm because odd circumstances exist (case in point, yes, Emperor is better than Leon in every-way at being a Dark Zone spamming machine, so Leon probably won't ever have to do that again, but not relevant here.) Self explanatory, so not giving personal reasons here.
Garland
Leon
Luneth (RD'd)
D. Cecil (go him?)
Seifer
Jecht
Zeid
Gabranth
Gafgarion
Like the previous list, self explanatory, moving on!
Now for characters with a bit more specific uses that don't really lump together. I'll preface that for a character to make use I feel they should have at least 4* in Black, Summon, Darkness or Ninja. I'd say "Witch" but given every single Witch user has Black 5, it's a completely worthless distinction. This list mostly highlights characters who could use Magic in scenarios where it's outright required for them to not use Physicals, like enemy has sleep or something. While it wouldn't be necessarily ideal usage and clearly under-performing compared to the previous two, it's just enough to be considered competent. This is why I put that 4* minimum; a character casting Firaja can pull off at least adequate damage, where as it's hard pressed to justify Firaga. Ninja has a low Magic Softcap, so raw magic doesn't mean much, just the Magic Sword helps hit that threshold with few buffs (and being ITD allows for a consistent minimum use, especially the 5*s), and in Darkness' case, Memento Mori is surprisingly effective on it's own, at least after the first cast, hence why it's there.
Galuf: Gets Swords and Ninja 4 via RD
Celes: Black 4
Basch: Gets Darkness 4 (through RD I think?)
Thancred: Ninja 5 (odd he's the only one with Ninja 5 and Swords near as I can tell)
Ramza: Ninja 4
Orlandu: Darkness 4
Lann: Summon 5, but his magic stats suck too much and Summon is a bit limited on it's own, and unlike, say, Garnet, his Bursts don't compliment Magic, hence why he's here.
So...yeah, I think this list covers basically all the characters who can make use of Magic Swords. Mostly thought of this because I remember reading someone claiming they're "uselses for anyone not Terra or Ashe." This list obviously dispels that!