I think the main reason is that some people live in zone 2 north - say
Kentish Town - but work in zone 2 on the other side of zone 1 - say Canary
Wharf. By buying a zone 2 paper weekly they hope to avoid the fare for zone
1 every day, which with Oyster they can't do;

They can't do it with paper tickets, eitherm when the Revinue Protection
crowd get ahold of them.

(Unless they - say - use the NLL to avoid zone 1 properly, but then
the oyster card won't charge them)

On the day prepay launched enough people with season tickets accrued
negative balances for the media to report it as a bug.

http://news.bbc.co.uk/1/hi/england/london/3422051.stm

There was of course no bug. All of those people did not have zone 1 on their
travelcards and for the first time the system was able to catch them out.