Would you pay out of your own pocket to take the CSSLP exam if you already have an ISC2 certification and you thought you could pass? What job opportunities or promotions have you found after getting a CSSLP certification? Why would you recommend the CSSLP over some other ISC2 certification if you were in an application security position?
It depends which ISC2 certification do you already have. For example next to SSCP or CCSP, CSSLP provides really good combo for DevOps position. Next to CAP or HCISPP is CSSLP useful only if you have deep interest to SDLC as a whole. I personally do not know any reason why to take CSSLP exam if you already have CISSP, but vise versa it is really good support for career move from developer position to architecture or management positions. As I have experienced then some global financial institutions have mentioned CISSP or CSSLP in their job descriptions as requirements for software development positions.
If you are looking in the AppSec space, I would certainly go for it, even if you are just looking for a way to differentiate yourself in the cybersecurity field. Having recently taken the CCSP (Would say gotten, but in the long holding period mentioned eldewhere) after getting my CISSP, I found that the "lesser" certs are useful in that they are more specialized. CISSP means a broad understanding, an expert in Security in general. CCSP means you know more about cloud, CSSLP leans you toward AppSec and Secure SDLC, which I can assure you, are needed. It won't get you anything by itself, you'll need dev skills, etc. but just look at the numbers; there are 82K+ CISSPs in the US, only 1300 or so CSSLPs, so it definitely qualifies as a differentiator.