How many pair of speakers do you own?

Eight pairs + one center channel. Of these, six pairs plus the center channel are in service in various rooms of the house. In addition, I have five subwoofers. (Not to mention incidental stuff like computer speakers.)

To answer your second question, none of your suggested answers applies.

In my case, (1) when I upgrade, I don't get rid of the old equipment -- they usually just get reallocated to other rooms; and (2) I think it's nice to be able to listen to good music reproduction in different rooms especially when it is also consistent with the decor. Do you have children? Are they into music? Should they have their own stereos, too?

Of my 8 pairs + 1, 2 pairs + 1 center channel are in my main system, which is both my best stereo and home theater system, and 4 pairs are in stereo systems in other rooms. (2 pairs are in storage.)