But you're wrong there - well sorta. The entire concept of AST is that it can fill the WHM or SCH role at a slightly lesser level, bit in return it adds the entire party benefit of it's card buffs. If you compare heal to heal or shield versus shield it will always lose. So now, there is a fight where Aspected Benefic would be needed to survive a hit, and the party has 1 WHM healer slot filled - who do you bring?

Answer: Whoever joins first or whatever your friend happens to like to play more. If a SCH comes, you will have more leeway after the big hit, and if an AST comes, you will have a little less leeway but all kinds of goodies being passed out throughout the fight. Either way - you win.