Given the way I defined a Bank-Account, you could implement this rather simply:
(def deposit (x . args)
(apply x 'deposit args))
Or, with akkartik's implementation, which I agree is nicer and better:
(def deposit (x . args)
(apply x!deposit args))
And you could make another "object" and give it a distinct 'deposit "method" and it would work fine.
So... it seems that one can easily implement these things in Arc. If you have some project in mind for which you want OO stuff, you can just do the things posted in this thread, and tell everyone how it goes if you like.
If you think OO should be put into the core of the language, well, there's a certain resistance to adding features just because someone suggests it. The core would become large, disorganized, and difficult to maintain if we did that. So, you would have to give a convincing argument as to why it's worth putting in there, and a demonstration of how useful it is in some real project of yours is probably the best argument you could give.