Turn it around: why would comics writers want to be part of SFWA? Certainly the Grievance Committee--the main professional benefit--is hopelessly compromised. However, SFWA membership might be a step in the process towards comics writers being treated like professionals, with, like, agents and other representatives.

Having access to the SFWA Legal Fund might be worthwhile, though in fact the CBLDF has done a good job over its lifetime in defending comics professionals. SFWA doesn't seem to offer health benefits, which amazes me; if they did, it would definitely be a plus.