i see many people dual wielding exotics. is there any reason for doing so? or does hpm work just as well if you only wield one and offhand wield, let's say, a dagger?

My understanding (which, in theory, reflects the code) is that you get the automatic defensive/offensive bonuses of HPM if the weapon in question is an exotic.

That is to say, if you're attacking, you're harder to parry if the weapon you're attacking with is an exotic. If you're defending, you parry better if the weapon chosen to parry (if you're dual wielding, this can be either weapon) is an exotic.

fds