There's likely to be a relatively long period of recovery right now, so taking a bit more time isn't a problem. But if the fighting intensifies in the future, and the Phoenix suffers heavy casualties, then replenishing personnel quickly will be critical. With the members of the Shadow Battle Group, it won't even take a few months for replenishment, allowing for seamless recovery.
Additionally, the Shadow Battle Group can do other tasks for Gu Hang.
When the Phoenix is too occupied or when there are tasks not even suited for the Phoenix... they will prove useful.
Interstellar Warriors are not exclusive to the Empire.
Those lost defectors, or even those who have fallen into Chaos, make excellent identities.
They're ideal for dirty work.
Some people die at the hands of 'traitorous' Interstellar Warriors; some deeds are carried out by defectors or Fallen. In any case, the blame can't be pinned on the Alliance, can it?