Actually, it's a good idea. In no way is it a invasion of privacy. Anyone
who wants to get some idea of a contributor's interests can look at his edit
record. There's no good reason not to have watchlists be public.
If you don't want people to know what your watchlist is, then don't have
one.
Wikipedia benefits the more open the actions and behavior of its
participants are to each other and to the outside world.
Qui custodiet custodies? Omnia mundi in Wikipediam.
Don't confuse this with actual invasions of privacy.