Quite how Nintendo can unbrick them is unclear. Regardless, choosing to risk this amount of bricking (I saw somewhere something like 1 in 1000 chance of it; I guess there's tens of millions of Wiis that actually get updated) is rather foolish if it's just to block homebrew. The existence of an unbrick ability is news to me; if your Wii does get bricked, I imagine they'll just send you a new one. If it's out of warranty, it might take some work unless a MS RROD style special warranty announcement is made.
Homebrew does enable some of the lamer versions of piracy, but the homebrew channel authors are fairly anti-piracy. It is harder to homebrew if you pirate as it gets in the way of the installer and the usage of the homebrew channel (they render the screen upside down if you have some IOSes hacked for piracy).
Mostly I use the homebrew channel for MPlayer (it can play videos from Windows shares) and SMS Plus (I like to choose from a wider list of games). I highly recommend trying it...