A few reasons why drow would want to avoid using humes/embrei:
1) drow live underground in high and in low places that require gear/manatech to navigate. Let us not forget the only reason Vaelia was able to remain a raider for so long was because of the fae she's been utilizing for years. I doubt grappling hooks will get you into places like the Beldrobbaen tower without notice. They may not be able to sense them, but patrols do have eyes and ears.
2) humes are not naturally stronger than drow, so they would have to undergo significant/special training to develop styles that would allow them to either overcompensate or use their smaller builds and speeds to take down much larger opponents.
3) That training requires time and money, and other races do not seem to have the longevity of drow. Within a blink of an eye the emberi would be useless due to old age. It's an unnecessary money sink no matter how promising the potential student.
4) The drow think anything not a drow is grossly unintelligent and incompetent, which enhances point #3.