Some people don't want to register with Amazon on principle.
I am going to resist the temptation to comment about peoples' preoccupation with privacy (suffice it to say I am happy registering
) but I will note I am jailbroken and use the collections manager despite being registered -- it's much nicer.
And there are hacks to fake a registration. Search the Developers Corner forum for details.
As for crippled devices, I don't believe the Kobo is usable at all without registering. Although there are hacks posted to write a fake registration to the database.
(The Nook is simply unusable.
)