Each type of plant takes certain nutrients from the soil...the ones it needs to make a good crop. If you keep planting the same thing in one spot, the soil will not be able to provide those nutrients. ALSO...each crop gives something to the soil, something that other plants really like.

Example:
Tomatoes love nitrogen and leach it out.
Green beans produce nitrogen on their roots. If you cut the plant off at the roots (or till the whole plant in after harvesting the beans), your nitrogen loving plants will thrive there.

Additionally, by rotating the crops, it is harder for the bugs/diseases to find them...if your tomatoes always live at the same address...as it were...they are easier to infect. By moving them, the baddies have a harder time locating them.

Basically, this depletion of the soil by planting the same crops over and over is one of the causes of the Dust Bowl of the 1930s. I certainly don't want to recreate that...even in one pot.

Here is a link that might be useful: Wikipedia Article on Crop Rotation

The concept is sound, but I don't think it really will make that much difference in a small back yard garden. If you move your Tomatoes 10 feet from where they were last year, the bugs will have no trouble finding them.

Now on the commercial large scale, it is important (notice the Wiki article is about large scale farming). The back yard gardener can pay a lot more attention to their garden and correct diseases before they get out of hand. It is also much easier to assure that your plants have the proper soil requirements in a small garden.

Bottom line, rotate if you can, but will your garden suffer terribly from "under rotation"? Probably not, but it might require a little more attention.

The worst part about not being able to rotate my veggies is the bugs that overwinter in the mulch. Since I live in an area that doesn't really get much cold, the bugs aren't killed off like in the areas that freeze. To try to deal with that I plant a fall garden and try to spread the different crops around. The backyard isn't big enough for everything I like to eat so I do tomatoes in the spring but cabbage in the fall. The peppers get replaced by spinach and so on and so on. Then if I get a freeze that does away with everything, I mix in compost let it sit until spring. Each time I change crops, I pull off the mulch and compost it so that maybe I can get rid of any bad bugs in it......the down side is I get rid of the good bugs too but I haven't noticed too much of problem.

The dust bowl was caused by sucessive years of drought and soil plowing over many millions of acres in parts of KS, TX, OK, CO and NE.

What Michael says is true. Click on the link below and then click on the years of 1934-1936 for precipitation and temperature....... Very interesting archive.

Here is a link that might be useful: 108 years of climate

The reason for rotating plantings in the garden is to reduce soil borne disease from infecting the same crop. Soil borne pathogens are plant family specific so rotating plants effectively reduces infection. This works great for most soil pathogens except fusurium which can persist for up to five years.

Pink root on onions can develop to an intolerable level in 3 years.