Well, why not? Land is expensive, and is the only base commodity that the entire human race has to share. Without getting too deep into arguments about the politics of land ownership, land is very important. Every person surely has some right to some land of their own. But take a look around any urbanised area, and you'll see that there is a lot of wasted land. Guerrilla gardening takes that land, and returns it to use. I'm not going to pretend that planting a geranium on a scrap of disused land is some huge Zapatista-like act of rebellion, but it is a highly symbolic act. And best of all, it's both fun and ecologically sound.
A quick start guide to guerrilla gardening
The basic principle behind guerrilla gardening is to take a piece of land which is not being used, and to grow something in it. So your first step should be to find some land that isn't being used. Look for grassy patches around abandoned buildings or deserted allotments. Grass will grow almost anywhere, and indicates fertile soil. The quality of soil can be improved if necessary by working in a compost or mulch.
A note on the illegality of guerrilla gardening
By now, you'll have no doubt realised that guerrilla gardening is perhaps a little on the illegal side. People, companies or local government will undoubtedly own the land which you are going to cultivate, and interfering with other people's land is illegal. Unless you can obtain permission to garden on the land in question, you should abide by three rules:
Use only land that is unused or unwanted
The land that you pick should be unused now, and for the duration of your vegetation's lifespan. Your definition of "unused" is up to you; derelict land is unused, but what about the grass verges on the sides of roads?
Leave the land in better condition than when you found it
If you're going to use some land for guerrilla gardening, then you should leave it in a better state than when you first took an interest in it. Improve the fertility of the land with compost, go organic, and clean away the detritus of urban life.
Don't get caught
This is as self explanatory as it sounds. In essence, you're doing something illegal on someone else's land, so don't get caught.
What to grow in your guerrilla garden
Chances are, with a guerrilla garden, you'll be working with a small piece of land in a largely urban area. You'll be growing in an area that the general public will not regard as a garden or farming area, so you should expect your garden to be ignored, or worse, trampled over. So, you'll need to use plants that are hardy, low maintenance and that have a high success rate if you want anyone to sit up and take notice.
If you want to extract maximum usefulness from your reclaimed garden, then I would recommend growing some form of vegetable or fruit and growing your own food. If it's shock value you're after, then a spray of bright flowers raised elsewhere from seedlings and transplanted out into the wild once established should achieve the desired effect.
One other plant that is often grown in a guerrilla garden is that activist favourite, marijuana. With the land clearly unwanted or owned by some corporate or government body, and tended by some mystery gardening-gloved masked hand, then who is there to prosecute? Indeed, can anyone be prosecuted for a natural plant growing in a (newly liberated) natural environment?
Political gardening versus gardening for fun
The very act of taking someone else's land and using it to your own end is in itself a political act. Whether or not you want your guerrilla gardening to act as a political message, with publicity and it own public impact is entirely up to you and the way your horticultural deeds are constructed. Planting pretty tulips on old industrial estates is one message, growing marijuana outside government offices is quite another. The choice is yours.