Not only do the Borg don't care about casualties, but they benefit from you trying to kill them as each successful attack gives them something to adapt to, making them more perfect and reducing their targets ability to resist them to a point where it is truly futile.
We've also seen that assimilation can be reserved by reasonably advanced species, such as the Federation. Infecting a person at range gives that person a chance to be evacuated and receive medical treatment. However, if you overwhelm a position and start assimilating there is no escape.
Edit 2: This is sort of an open ended title I guess, so maybe we can come up with other seemingly obvious things they should have done.
In that case, why didn't the Borg send 150+ cubes into Federation space, one for each member world? Starfleet would never have the numbers possible to defend against each one.
Given the distant from the Borg territory in the DQ and the Federation's technology level there probably isn't a strong reason for the Borg to launch a significant effort to conquer the Federation. However, the Federation is advanced enough/interesting enough that sending a test every now and again would be worthwhile.