Indeed - players in other modes don't get to perform direct Powerplay activities, merely peripherally affect Powerplay via the BGS with none of the Powerplay rank rewards.
.... are those players expected to remain engaged in a feature that they don't receive rewards for or might they just do their own thing with respect to the BGS regardless of how it affects Powerplay?
... as it stands they would receive far more income by continuing to supporting the BGS factions they'd become allied with, and yeah, they would remain engaged. A lot of players come to powerplay because theyve achieved their individual goals, and/or want to do things to help a cause.
Theyre not put off by their Power's NPCs calling them scum regardless of rank or time-served. So why would they be put off by having a rank or not, especially when it is only visible to you, and have to go into a side panel, a menu, and then two further sub menus to actually see it!
Besides, the OP isnt proposing OpenOnly. The OP allows for full status & ranks for all pledges in all modes, regardless.