I don't use NBU for Exchange backup, but I bet my 
head that it is the same thing as Legato.  This 
means following:
- if You send 100 same messages due to SIS inside
  IS You will have 1 message and 100(-1?) pointers.
- when backing up IS (and DS if using 5.5) You will
  get this quickly as MAPI is not used
- when backing up mailboxes individually, this will
  use MAPI and it slooow and what it does is that it
  extracts these 100 messages so at the end You will
  backup 100 mboxes with 100 same messages (each messages
  for one mailbox).  This is due to MAPI limitation.

So to Your question "What I'm being TOLD is that if 
you send an email with an attachment to 100 people, 
NBU will back it up 100 times.  Is that true?" I 
would say yes if we are talking about mailbox backup.

Think for a moment; if not the pointers should be
backed up separately and they should be read by
backup software to see if that mailbox has any
pointer and the restore and bla bla... doesn't
make sense.  I

Legato limit You mention is due to following:
- when You backup 30 mailboxes they are all extracted 
  first to nsr\tmp and if these mailboxes are big
  they may exceed Your disk space (now You can specify
  some different target for tmp so You may override
  this limitation).

I hope that Microsoft will change one day their
approach regarding the possibility of mailbox backup
for the sake of everyone.

The best approach I saw so far is to have scratch
server and recover on it IS and from there run 
mailbox sync from outlook.  That's the fastest way
that I know of single mailbox recovery.

Cheers,
Hrvoje