if you were to say, take a cleric to level 9 and then advertise for a new player run of the pit elite. how many raise dead scrolls do you think you'd use whne the majority fo the people in the party have 70 hit points. now think about it from the perspective of the new player - dying is frustrating. it costs the party 10% xp, you are dependant on someone else to get you back alive (IF they get you back alive).
now, speaking to your bard example.
(assume 14 CON)
base bard at level 20 (120 Base + 20 HD + 40 CON + 10 Argo) = 190
base bard at level 10 (60 Base + 20 HD + 20 CON) = 100
(assume 8 CON)
level 20 (120 Base + 20 HD -10 CON + 10 Argo) = 140
level 10 (60 Base + 20 HD - 5 CON) = 75 HP