The reason it will not do that or install your /home on a different drive is that the installer is just pretty lame.
As good as the rest of the OS is it really desearves a better installer.
I do a lot of Debian istalls from the netinstall disk. Doesn't get much more manual than that. I like manual installation.
If you have a very nice Live CD to install from though it is a great way to install because you can spend your time doing something on the box while installation takes place on another workspace. You can listen to music with a little for thought about getting some before starting to install. You can explore the new OS and its configuration and have most of that all figured out before you ever get booted to your new install.
LMDE has a great Live Session. It really, really needs a better installer.
Dell XPS 420 Core2 Quad Q 6600, audigy5.1, Radeon HD 6450 - currently 4 320Gb HDD, Debian Squeeze for secure use, Debian testing for daily use, Debian Sid for fun.