While AST is certainly serviceable in 50 content, from a trash compacting standpoint you get significantly more mileage with Holy (or even DoTs + Bane from a SCH) versus an AST with the only AoE option being Blizzard II. WHMs make the fastest runs in 50 since their upfront DPS with both AoE and single target in max ilvl gear helps melt things excessively quickly with little concern for their MP.