Mere pirates couldn't be much of a military asset, especially against a force as mysterious and powerful as the Scarlet Eyes. However, they had other uses.
First, the pirates had connections to illegal endeavors, which were linked to criminal organizations. In the past, Khan had dealt with the Hive, Mister Chares' force, and whatever group Raymond Cobsend was part of, and they all had managed to create problems for him, vouching for their power and resourcefulness.
The universe was bound to have more of those criminal organizations, especially if Khan expanded the search past humankind, and they could help in countless ways. Their information network, access to rare materials, and overall knowledge were just some among many tools Khan could deploy during a universal war.
