I don't encounter many hackers while playing, and judging by the recent bans on the Wiimmfi page, hackers are usually taken care of pretty efficiently. I suppose you encountered something fairly rare in my opinion.
I believe Wiimmfi also has a system where is requires you to wait 7 days before playing on a new console as to prevent a huge surge of returning hackers (and reducing the flow of potential hackers in general too).
So within this system, you'll see one once in a while, however I don't believe this will be a consistent problem.