Assembly language is good, infact i done a control program to a factory using pure assembly the other day, I could have used C language but I felt Assembly was the way of doing this sort of PLC control.
My program is still running and produces milions of meters of irrigation pipes every week.
I agree that Assembly language is un beatable when it comes to tight timing and code space constraints. And there are good programming practices for both Assembly and C languages, mixing the 2 together you get the best out of both worlds.
I find it easier to write in Assembly for shorter simple control programs, and easier in C for complicated flow control programs with data structures.
To be honest I never tried to mix the 2 languages together, If I needed fast response time I always write in Assembly.
I guess it all depends on the application and there always are lots of ways of doing things.
Regards
Mahmood