I cannot accurately attest to what percentage of BS informed the OBS. My best guess, however, would be a super majority.
One practical reason for informing the OBS is that affairs cannot thrive in the light of day. Once exposed, continued secrecy becomes increasingly difficult.
Another practical reason is that a BS gains another source of information. While not perfectly reliable, because WS will lie, sharing information can, and usually does, expose those lies.
Speaking philosophically, morally, I honestly believe we all have an obligation to look out for one another, even strangers. By informing the OBS, you've done a "good deed." You've helped them by giving them back their agency.
For me, personally, it went a bit further, and this was my deciding factor. By keeping the affair secret, I became an accomplice, which was not something my conscience would allow.
In my situation, the OBS thanked me for informing her. I'd sent her a letter by registered mail detailing everything I knew and how I knew it all. I included our names and address, and my contact information (phone number, email, and even Fakebook info).
Did telling WS's AP's spouse have any positive or negative impact on your own reconciliation process?
That's difficult to assess for the simple reason that I cannot know what might have been different had I made a different choice. What I can say is that once I told my exww about it, after receiving replies from the OBS, she was forced to accept that not only had she hurt me, she'd also hurt a complete stranger (although she knew AP was married).
Also, I think, it forced her to accept the consequences of her actions. The husband she had before was no longer going to protect her at any cost. In that respect, I think informing the OBS was a net positive.