Earlier quoted context omitted.
In 99% of cases, ppl didn't reveal their private email address. NOW, people can infer your facebook email address from your facebook id... thus making spamming you much more likely. Facebook IDs are public, right? You can argue people can already mass-message people, but now it's easier.
If you compose an email to multiple @facebook.com addresses, I'm sure Facebook will limit the actual number of recipients in the same way that they do when you send messages through the Facebook message interface, so it's really no different. With 3rd party spam, the advantage Facebook has here is Zuck's "social graph" -- they can pretty easily tell what the relationship of the sender is in relation to the recipient…
You think FB isn't in (or aspiring to) negotiations with ConstantContact, CampaignMOnitor, MailChimp, etc.? Triangulating preferences, deep dives, data warehousing, you name it.