More white whites = more chances at a crit = more uptime on flurry.
Yes the cleave hits weaker, but you get more cleaves in a lesser amount of time.
HS and cleave add a base a mount to your white hit for it's damage calculation.
Heroic Strike (IX)
http://db.vanillagaming.org/?item=21297Example:
SWORD CTS (58.5 DPS) - Average damage 152.1
Heroic strike (IX) adds 157 to damage. My 2.6 sec weapon does 152.1 white damage (58.5 dps). I do 309.1 damage per heroic strike. I can do 23.07 Heroic strikes in 1 minute with the 2.6 second weapon.
23.07 heroic strikes = 7,130.93 damage. = 118.84 DPS from Heroic strike spam.
DAGGER Perdition's (58.3 DPS) - Average damage 104.94
Heroic strike (IX) adds 157 to damage. My 1.8 sec weapon does 104.94 white damage (58.3 dps). I do 261.94 damage per heroic strike. I can do 33.33 Heroic strikes in 1 minute with the 1.8 second weapon.
33.33 heroic strikes = 8,730.46 damage. = 145.50 DPS from Heroic strike spam.
Result:
You obtain a 22.42% damage increase from Heroic Strike (IX) using Perdition's 1.8 speed vs CTS 2.6 speed.