...and yet I know a ton of people who have fully-private Twitter accounts, every one of which who bemoan the fact that they don't /want/ to have a /fully/ private account, but that they are more comfortable sending an important subset of things they say to people they know, an are therefore stuck.
These people then end up having multiple Twitter accounts, which is a drastically more difficult "extra step" to perform when they go to post a new message: sufficiently so that a good number (most, in fact) of the people I know who do this simply don't bother, and /only/ keep the private account.
(Although, I personally am in this overall category, but chose to keep only the public one, and thereby simply "don't really use Twitter, as they apparently only care about weird totally-public PR garbage".)
If Twitter added the circles feature tomorrow, people who actually considered it a pain to think about simply wouldn't: they would ignore the box, and business would go on as usual. However, if they wanted the feature, they would probably start using Twitter a lot more; and, as they are no longer "rapidly" switching accounts before posting, that allocated time would be even more effective.
(Also, for the record, if Twitter did do this, it would be even extra awesome, as "listing" someone already provides all of the "circled" data model, meaning that a lot of people, like myself, for whom this feature would have immediate value, already have our followees organized usefully.)