A matter of practicality. Each instance is joined to a couple of channels.
I could join them all, but the point is that I tend to be in 8 +/-2 channels in both gimp.net and freenode.org. Plus a couple in the private network. Switching between more than 10 channels in irssi is too much of a hassle for me, more than 20 channels is unpractical.
Plus, this way I can label the screen windows (Freenode, Programming) or (GIMP, Gnome development) and have a better way to context switch between completely disconnected things.
I could join them all, but the point is that I tend to be in 8 +/-2 channels in both gimp.net and freenode.org. Plus a couple in the private network. Switching between more than 10 channels in irssi is too much of a hassle for me, more than 20 channels is unpractical.
Plus, this way I can label the screen windows (Freenode, Programming) or (GIMP, Gnome development) and have a better way to context switch between completely disconnected things.