A bad cleric does not watch red bars, forgets to turn on his aura, does not have the DCs to do any real casting etc. A hireling then functions better.
Furthermore playing a "good" divine is expensive, not to be underestimated for a first toon. Buying 100 Heal, scrolls 50 raise scrolls and a bunch of wands drains your money pretty quick. My first toon was broke on several occasions.
Agree with most except the DCs bit - most Clerics have no hope of landing spells in Epic Elite due to SR, let alone DCs. That doesnt make them a bad Cleric.
Most general PuGs just want a healer - being able to CC/ Insta is just icing to many PuGgers.