That's what I thought, then I read this:
The two are not the same. Defender is purely anti-spyware. MSE includes this functionality and also includes full antivirus protection
However, that answer was from 2009, so your post forced me to read further and brought me to this answer from the same thread:
It *was* the whole story in 2009 -- when this was asked and answered before you resurrected it from the archive!
Microsoft, in their infinite wisdom (sarcasm), chose to use the Windows Defender name for 3 distinct products -- only one of which existed when this question was asked in 2009.
Windows Defender (downloadable for XP, included on Vista and Windows 7, antispyware only, includes some useful system utilities, too, not present in MSE or Windows Defender/8-RT)
Windows Defender Offline (uses the same technology as MSE, but runs from a bootable disc created by the downloaded "wizard")
Windows Defender (included in Windows 8 and Windows RT that is a full antivirus and antispyware program)