Once, probably never again. Their utility is far too limited. Pretty much any role they fill in a campaign can be filled by another group more effectively/believably (ex.: the Society, Belters, any group whose members use implants, basically any group).
The problem is that the genecaste are a reclusive faction with limited political/military ambitions. Their territorial expansion is usually into areas where most other groups tend not to go (usually due to environmental conditions). Those who interact most with the outside world have undergone the least genetic manipulation and thus are effectively no different than a standard npc with one or two implants. The most senior of the group (aka those who have undergone the most genetic manipulation) are extremely reclusive and, on occasion, are limited to particular environments due to the extreme genetic manipulation thus making them as rare, if not rarer, than senior MD.
By the time the players realized they were being manipulated by the Genecaste (due to the introduction of a more senior member of the Genecaste who was beginning to show visible signs of genetic manipulation), the immediate storyline was almost over. When given the option of pursuing the Genecaste and regaining some of what they lost (pride, resources, their pay), the players opted to move on rather than spend more time on a group whose participation in the campaign so far was lackluster. It didn't help that the indications were that it would cost the players more in the long run to continue the storyline than they could possibly hope to recoup from the Genecaste.
