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.