Erik,
It's not that it "can't," the fact is it doesn't. There's a somewhat different paradigm concerning what is considered a "group." In Address Book and Mail, it is a single "Distribution List." That list includes one email address for each contact.
The paradigm goes further, with the assumption that the "To:" field of a single email can be populated with multiple contacts or multiple groups (or a combination of both). In many ways, this becomes a more flexible workflow when composing messages.
OK, perhaps it doesn't work as well as something else might
for your specific application in this case, but it would be more functional for other situations. I'll give a great example...
You are a public school teacher, and you are "District Chair" in your field's regional professional organization. You will have the colleagues with whom you work at your school, but then you will have many others
in your same field employed at other schools across your region. As District Chair, you frequently communicate with this second group of individuals via email. You know all of them personally, and consider many "friends."
All of the individuals in this second group, which you could be considered to "support," will have multiple email addresses, and you necessarily must have at least two: a "school" email address, and a "home" email address. In such a scenario, one thing that is ever-present in your mind when sending out messages is that you are sending to
official public school email addresses.
Now, let's suppose you come across something pertinent to your profession, but it is humorous, perhaps satirical, and not something you would want to send via "official" channels. This could just be something funny that you wanted to forward to your District members. If your only "Distribution List" contained every email address of every contact, you would be stuck. At the very least, sending this one email would require an onerous amount of work.
On the other hand, the current Address Book/Mail paradigm works quite well for this, because your District members are included in
two Distribution Lists: one for their "home" email and one for their "school" email. If the message you want to send is only appropriate for one or the other, you're set. If it must go to both, you're set.
So, I'm going to recommend again that you set up 2 groups (or more, if called for), and populate those groups according to
type of address. I won't entertain the idea that either typing or dragging two separate groups into the address field in Mail is an onerous task, or a "workaround." It is neither. 😉
Scott