It appears an assignment always returns true!
Not that I expect this to improve your perception of C, but the value of the assignment expression a=b is actually [i]b[/i]. So your observation is correct except when zero is assigned.
Back to the question about why && when we could have AND or % when MOD is available: Admittedly this is a matter of taste, but I find that reading well-formatted C code is easier than most languages, precisely because numbers are numbers, names are names, and operators and delimiters are, well, these other little squiggles. It wasn't long after I migrated from Pascal before I started to enjoy the succinctness and readability of {} versus begin/end and % over MOD. Hopefully you will too!
Mikal