I think this is just might be one of those things which would ticks us off as techies... but techies are the minority audience for sites like facebook.
Do emails sent to the facebook id go to the standard facebook 'message system'? If so, for a majority of people this will probably make sense... want to message somebody on facebook? You don't have to login to facebook! Email them at xyz@facebook.com. I didn't know you could do that, pretty sure a lot of other people didn't either. Now a lot of folks probably are updated about this feature.
In addition: non-techies who have accidentally exposed their private email id's will now have their facebook id's scraped instead... and the ones who exposed their private emails intentionally will change it back anyway.
However, all said and done... it could have been done differently.