There are many reasons for not doing this and many of the arguments are the same for any genetically modified plant or animal to be used or consumed by humans. These include:

1) Safety: Potential human health impact: allergens, transfer of antibiotic resistance markers, unknown effects potential environmental impact: unintended transfer of transgenes through cross-pollination, unknown effects on other organisms (e.g., soil microbes), and loss of flora and fauna biodiversity.

2) Access and Intellectual Property: Domination of world food production by a few companies; increasing dependence on Industrialized nations by developing countries; Biopiracy foreign exploitation of natural resources.

3) Ethics: Violation of natural organisms' intrinsic values; tampering with nature by mixing genes among species; objections to consuming animal genes in plants and vice versa; stress for animal.

4) Labeling: Not mandatory in some countries (e.g., United States); mixing GM crops with non-GM confounds labeling attempts.

5) Society: New advances may be skewed to interests of rich countries.

However, on the upside, silk proteins are truly remarkable materials and could be used in a wide range of important applications where high strength, low weight, tough fibers are required.

Other benefits of genetically modified plants and animals include:

1) Crops: Enhanced taste and quality; reduced maturation time; increased nutrients, yields, and stress tolerance; improved resistance to disease, pests, and herbicides; new products and growing techniques.

2) Animals: Increased resistance, productivity, hardiness, and feed efficiency; better yields of meat, eggs, and milk; improved animal health and diagnostic methods.

3) Environment: "Friendly" bioherbicides and bioinsecticides; conservation of soil, water, and energy; bioprocessing for forestry products; better natural waste management; more efficient processing.

4) Society: Increased food security for growing populations.

You decide...Does the gain outweigh the potential costs involved?