You can't compared this game to XI, this is where your argument fails, you are comparing a Conjurer, to an XI WHM which had a lot more enhancements on hand to think about, where protect/shell had it's long duration because of all the other enhancements they had to focus on. In the same sense you can't compared Rampage/Berserk to it's XI counterpart because in 14 these are stances which you can only have one active at a time whereas in XI they are self buffs.
As for the reason why Shell is Conjurer only it's so if you want magic defense you need to bring a Conjurer along or else just deal with a physical protect with no magical defense added to it.