I have heard two competing ideas:  the idea of NOT planting intensively and spreading plants far apart as a way to maximally utilize water and nutrients VS. planting tightly for some of the same stated reasons!  I get the idea, though, that mixing root zones is important.  I'd like to hear more thoughts on this.  Go ahead -- confuse me!

Pennie

First, check out the book "Square Foot Gardening" - if gives you a really good idea of spacing for each plant.

Next - check out "Carrots Love Tomatoes" - this covers companion planting.  The whole subject gets much richer than planting shallow rooted stuff next to deep rooted stuff.  Some plants sort of try to kill each other and some plants try to help each other. 

For veggies I would look at what size they are when you are putting them in the ground vs their maximum size. For example, between cabbages you can pull of one or two rounds of salad greens or radishes before they get up to size. This would maximize production (intensive).

If you don't need/want that much production and you would prefer less maintenance, I would plant things at their adult spacing and mulch with grass clippings (or something else easy to remove) between them until they shade the entire bed. Another advantage of this is that you can still use mechanical weeding tools (stirrup hoes, diamond hoes, etc) between your plants since the bed isn't packed with veggies. Either way, I would highly recommend making sure your soil is always covered (with mulch, cover crop, or anything to shade. By shading your soil you will minimize watering needs and increase soil micro-life (by as much as 30%!).

Thanks to both of you.  I have checked both of these books out of the library, but I keep hearing them referenced and get the idea that having them readily on hand (and reading more intensively) would be helpful. 

I have been using burlap between rows and on temporaily unused areas and have been liking that, at least in my neonatal gardener stage.  I'm trying some grass clippings, too, though ours tends to get high and weedy (and seedy).

Pennie

I believe the whole normal spacing and intensive spacing has alot to do with how much space you have to grow and what your goals are.
If you live in the city on a small lot and dont have alot of space you may have no other choice but to grow intensively (meaning spaced closer together)
Or if you live out in the country and have plenty of room but want as much production per square foot or per acre then you could choose to grow intensively. Such as maxamizing your dollar per acre output.
I for one have plenty of space and am not that interested in dollar per acre so I can spread out more.

There are pros and cons to each system
Spacing the plants closer together keeps down the weeds and shades the soil. But it also uses more nutrients and minerals and water so you need to water more often and apply compost or fertilizer more often.
Spacing the plants normally allows more weeds to grow (unless you mulch good) it allows the roots and plant to spread out more getting bigger and uses less water, minerals, and compost, and fertilizers.
Either way you don't get out of working the garden.

I see two reasons for choosing different spacings between plants.

First, you can't really place plants closer than what they need to grow.  You can visualize a 4ftx4ft square planted with 16 large-head-type cabbages.  If the heads get 10 or 12" across, and you plant them 6" apart, by the time they're partly grown, they will be too close together and shading each other out, limiting how much sunlight they get. If one is stronger than the ones next to it, it may grow large  because it gets the most sunlight for the most chlorophyll production, but it will be shading the others out and they will be smaller.

The second reason is the nutrient level of the soil. If a plant is going to grow to it's maximum size, it needs a certain amount of nutrients. If the soil is excellent, with all the nutrients, and those nutrients are available in the form the plant can use, the root system doesn't need to go too far to get what the plant needs, and you can plant them relatively close together, as long as you allow enough room for the plant to absorb enough sunshine for it to grow well.

But if the soil is poor and the nutrients are spread thinly through the soil, the roots of each plant will have to spread out further to find enough nutrition to grow.  If you planted them close together, they will be competing with each other and none of them will get enough nutrition to grow well.

It all depends on your soil and the available light. 

The more you garden, the more you will learn.

Sue

I think alot depends on whether you are gardening for "survival" or gardening for supplementation. If you are growing vegies because you need the food then you have to consider what would happen if you didn't have ready access to supplemental water and fertilizer. if you are trying to maximize yeild for the space you have so you can  provide food of superior nutrition than you already ahve access to then intensive managment makes more sense because if neccessary you can fall back on the other food source.