Why does mIRC always returns 0 when I do //echo $idle ?
Have I got to use raws in order to retrieve information about my own idle ?

Because after you type the line mIRC resets the idle counter. You can use $idle in a script without it resetting.

just to demonstrate it, try doing:

thatr should return 5, assuming the timer is correct.

Try
//timer 1 5 /echo -a idle = $!idle
instead.