The plant roots grow into and attach to the polystyrene and cannot slide out. The cells then have to be broken in order to remove the plants. The wind blows the pieces ALL over the garden! They can't then be re-used and therefore have to be rounded up and thrown away! Why are they used at all?
The only possible use I can think of for broken bits of polystyrene is to put in the bases of large pots (therefore reducing their weight) for drainage instead of gravel but I can only use so much!!!