If any of your components are going to have fast switching times (short risetimes or falltimes), and if your board is going to have to meet EMC (electromagnetic compliance) rules, then you absolutely NEED power and ground planes.

Yes, even the simplest systems will heavily benefit of using at least solid ground plane. Of course, one could route the massive ground tracks all over the board by hand, if there doesn't exist this 'ground plane manager'. But that's not the issue. You must also route ground tracks between all the pins (assumed you use throughhole parts). And this is rather impossible doing by hand.

Many people totally underestimate, how much interference is caused, if even only one output signal of 74HCMOS gate is not routed properly over a solid ground plane, means if signal line and ground return current form a loop.

There's a very nice demonstration here:

http://www.murata.com/emc/knowh.../5to7e.pdf

Only one output signal of 74HCMOS gate routed unproperly can make your application fail the CE radiation test!!

Kai