A shield equaling 10% max HP is functionally the same as a 10% max HP buff that heals that amount, you essentially get a temporary cushion of 10% max HP to soak damage.
Doing it as a shield basically streamlined it by removing the two-step approach of an HP buff and a heal, consolidating it into one effect and removing any other potential complications such as regens/medicas affecting the HP buff ability during it's duration and increasing its potential potency.
While asking for specifically an AoE shield wasn't common, it is pretty easy to see the translation of the asked for ideas and what was provided.
Also context matters. At the time there were a lot of tank complaint threads going on involving DRK and WAR. There was a general consensus among many if not most tanks weighing in on the issues that both WAR and DRK should get some form of AoE/Partywide utility, hence why something like a 10% HP shield didn't seem imbalanced, because all the tanks would have something fairly equivalent.
So yes WAR mains as well as other tanks in the community DID ask for SiO to be changed into an AoE utility skill akin to what we got, as well as asking for DRK to get something too. The problem was that WAR got many of the things and considerations being asked for and quite quickly and comprehensively while DRK did not get an AoE utility ability along with many of the other issues/things asked for addressed. Giving to one and not to others created the imbalance.
EDIT:
Back on the topic of PLD AoE ...
One potential issue that I see with consolidating down the AoE abilities into one that does damage and enmity is in situations there are multiple tanks, like alliance raids, and the OT PLD is spamming their AoE ability and potentially stealing threat from the MT. Then again that is really no different than DRKs and Abyssal which isn't really a problem, that and such situations are rare and the impact is pretty negligible.
Anyways, just mulling over this stuff a little.