depending on your AD size and OU structure, there well may be 100s of accounts who will NEVER require scanning to their email but will still get listed i.e. the user at the machine might have to scroll through them.
I have set this up at a couple of sites and cannot see any other disadvantages.
Some copiers have on screen buttons for which you can setup by name which will be better in my opinion than having to type in the email addy everytime. Also the way we have ours setup it pulls from the GAL by typing in a portion of the users last name and then you can add the button for them. The only draw back I've seen here is keeping up with removing those that have been added that are no longer with the company. Other than that I've had much success with it and love it.
Also make sure you take a look at your default file size or you might run into situations where larger files that are scanned get cut up into smaller files and you may not be able to open them once received via email.