I can't think of a more useful plugin for a Track FX in a portable device: a noise gate would really help control inter-channel bleeding (better handled at the recording stage) as well the ambient noise floor in certain environments. IMO it's an essential one for Multitrack DAW.

Agree with this, a noise gate would be really good.

Not a bad idea however;
And Pwnified can correct me if I'm wrong, but I seem to recall that the compressor has a built-in downward expander. You just have to get it set right.

Also,
If you have Jam-up pro there's a great gate built in.

Not sure what inter-channel bleeding is unless you are talking about mic bleed as in drum set miking. If you are having that much of a problem with mic bleed, it would best be handled by a hardware rack gate before getting it into the digital domain.

Marty,
Isn't downward compression just lowering the volume of loud parts? How does that solve the need for a high-pass filter or noise gate?

Downward expansion detects when the signal starts to fall below a certain level and then acts like a smart gate. Not as harsh or sudden as a gate but good ones work fairly transparently. They work great as long as the noise floor is fairly low, like -75dB or lower.
Some times you can eq out some of the noise floor in order to allow the expander to do its job more easily.

Compression and downward expansion took me forever to understand.