Haha RDM is such a versatile job I'm not surprised there are such differing opinions on what a RDM does/ is good at, it's good to see^^

There's a difference between an actually essential part of a group by game design and something with an inflated value due to a handful of factors. RDM, like it or not, has always been of the latter.
Your kidding right <.<? RDM has always been and still is an essential job in my HNM LS -Abyssea LS is probably more accurate now, who does HNM any more? lol- De-buffing mobs makes a tanks job so much easier & on some mobs took them from challenging to easily manageable, enhancing was also always a big part of my job at events allowing our melee or mages to pump out more damage while I also helped to keep them healthy. In a party or alliance situation these have always been a RDM main duties as compared to other adepts in each class RDM DD is lacklustre (that's not to discount RDM offensive capabilities though which solo can be quite good this coupled with RDM excellent survivability make then in my opinion the best soloer in the game & this was always how I enjoyed playing the job most).

Blue Mage has "better" "AoE" buffs and a better skill.
BLU doesn't really have any AoE buffs except SS unless you use Diffusion & even capped -& why would you cap this over other things?- with the low duration of most BLU buffs it's pretty useless.

I mean the VAST majority of Rdm's spells have actually been self-targeting only.
True but that doesn't limit us to 4 buffs /whm we get all the important ones & you know SE likes people to use their subs.

Exactly. Whm can buff everyone at once. They can get pro/shell up on a party without spending 1k mp, and a minute of casting.
/SCH & you can pro/ shell the entire party at once.

Enhancing Seal seems like an idea waiting to go wrong. Last thing I need is a bunch of DDs complaining because I used the seal + haste on someone other then him.
Lol I could see this happening to.

RDM only Refreshga 1-2
Go /SCH

We've been given spells that other players want casted on them. We get only single-target versions, which means we spend most of our time casting these single-target spells, robbing us of a lot of time
This is what Composure & AF3 are for.

I don't really agree that RDM needs big changes to make it a relevant job for buffing again & I think the changes the OP is suggesting while sounding nice are kind of game breaking. RDM is a jack-of-all-trades master of none -save enfeebling- always has been. We get a lot of the benefits a SCH does using it as a sub now, sure Accession triples recast but with how long our buffs can last now who cares? Even though we have the highest enhancing magic skill we were never in any practical sense THE enhancers we were always supplementary in the role helping the other support role jobs enhance the party with de/buffs and cures. We will never beat out jobs such as BRD that are specifically geared towards party buffs because we have so much skill in other areas.