the default case would be optional..
yes, default would trigger if no other case statements were used or accessed, You could also use it as an 'ending' or 'closing' statment too (if you omit a break from a case statement (or all), it will continue to check, and hit default, and execute it)
There are many reasons for adding a switch/case statement and compatability in mIRC, there's also many reasons not to, but i think there's more reasons to add it, and many people would love it..