To be exact, some identifiers (but almost all directories identifiers) accept trailing text, it is very common to see in script something like "$scriptdirtest.txt" where test.txt is appended to the value of $scriptdir, just as though $+ were used.
$scriptmovie is naturally just $script $+ movie for mIRC, so this is a limitation.
Note that mIRC supports two custom identifiers prefixes '.' and '/' which allows you force mIRC to call the custom alias: if you had an override of the 'me' alias, $me would still call the built-in identifier while $.me or $/me will call your custom 'me' alias. These prefix allows to workaround the limitation; $/scriptmovie will call your alias, but it's not so good practice to use that, renaming the alias to something else would be better.
