One of the reasons that we always build websites that use email address as the username is because it's easy for the user to remember. I've just remembered another reason – if your website becomes seriously popular, you run out of good usernames.
I just tried all these names while trying to register on Last.fm (Alexa 344):
jake
brumby
jacket
orangutan
simpleminds
nenehcherry
joshuatree
fishandchips
monkeytoes
dogbreath
giblets
chickenfeet
chickenfoot
monkoid
Spatula
frangipan
mugwump
flobadob
jinglejangle
qwertyuiop
asdfghjkl
yoyoyoyoyo
thisiscrazy
crazyusername
nomoreusernames
iamstuck
aaaah
aaaaahAnd then I found it…
aaaaaah