I'm not sure. I wasn't working as a kernel developer. I did have to debug stuff though to find bugs or explain certain behaviours.
I guess one reason it wasn't used much is because it was a hassle to set up, and I think it got a bit slow when you had a lot of information displayed. I do recall not using this setup much in practice, as just breaking into the debugger on the machine itself was faster and easier.
You had to lot of options when it came to working on the Solaris kernel, and the code was so easy to understand. In fact, I believe the Solaris code base is the best C code I've ever worked with. It's a pity what happened after Oracle took over.