Graves are not the only way to get rid of them.
They might be handy in early game, but as attacks become more frequent and larger, you absolutely want an option which takes less labor and space.
The primary motivation to get rid of them, is the malus your people receive after seeing a corpse. This malus increases, as corpses decay. This malus stacks, so a field of decayed corpses can easily provoke a psychic outbreak.
The ways to get rid of dead bodys are as followed (Updated at Beta 0.19):
Burying corpses will prevent the dead body malus.
However, this is quite labor intensive and takes a lot of space. Not suitable for mid-/lategame.
Requires research "Electrical Crematorium"
Crematorium is an appliance to destroy certain objects including corpses.
Operating the crematorium is done by haulers.
Use them for psychological warfare
While this has some major drawbacks, setting up a designated area to deploy corpses, without intention of further action, can also be a strategic trick.
A storage full of corpses at the entry of a kill-zone can make invaders go on a rampage and attack their own mates.
However, the hauler carying new corpses to the storage, will suffer mentally on a regular basis. Even psychopaths receive a malus, when they are around decayed corpses.
Process their meat
By butchering an human corpse, your entire colony will receive a, long lasting, -6 mood malus ("We butchered an human"). The traits Cannibal, Psychopath and Bloodlust will prevent this.
Eating human will even go further and add another -20 malus.
If you can handle a general -6 malus for your colony and your cook is a psychopat, cannibal or has bloodlust, you can use human meat to produce
Kibble for your animals. They couldn't care less for their protein source.
Dead colonists should always be interred, as other ways of getting rid of them will produce mood malus in your colonists (Except psychopath)