The first part of that theme is indeed true - rainforests and other biodiverse areas are potentially rich sources of new medical and biotech resources. And, indeed, this is one argument that's often advanced by conservationists; the argument that rainforests are of great instrumental value to humans beyond the value of their timber, and the source of that value should therefore be conserved. This is one of many arguments thta one can advance in the name of protecting rainforests from malignant economic practices like logging or clearage for agriculture.
The second part of the basic theme - about bioprospecting as an economically viable conservation tool - is much more complicated. Another post from 2006 explains,
...Some firms maintain that natural products discovery through genetic prospecting remains an important form of research into the indefinite future. Others at the forefront of the industry, however, suggest that genetic prospecting or natural products research is much less important than previously thought (Service 1999). Virtually all drug development proceeds by “rational drug design,” involving computer visualizations, laboratory production, and high-throughput screening. Genomics and combinatorial chemistry have radically altered the drug research landscape. Instead of designing drugs through testing and altering natural chemical samples from organisms, drugs can be developed from an understanding of how genes and proteins behave. Some researchers have contended for some time that combinatorial chemistry could render genetic prospecting redundant (see interviews in Macilwain 1998; Service 1999). Some university-based drug development chemists contend that such technologies render prospecting for samples from natural products at least less significant; at best, promotion of these technologies and methodologies is cyclical, built largely around cost-effectiveness, institutional inertia, and the potential value of natural products research versus the shorter time horizon of developing drugs through rational drug design (private interviews, 2005; see also Olsen, Swanson and Luxmoore 1997). Olsen, Swanson and Luxmoore maintain that “perceptions of the difficulties associated with [natural products research] appeared to lag behind technological advances which would significantly reduce those difficulties” (1997). One wonders, however, why perceptions do not seem to have changed much over more than a decade. Others suggest that the best approach is a balanced combination of rational drug design, combinatorial chemistry, and natural products research (Lahana 1999). Finally, even if genetic prospecting does remain an important element in drug development, research on extremophiles (organisms inhabiting severe environments such as marine thermal vents) and microbes in environments such as toxic waste dumps indicates that there is no guarantee that genetic prospecting and its benefits are ineluctably tied to biodiverse areas.
Biodiversity's marginal value may indeed be negligible, while new drug development technologies indicate decreasing industrial dependence on genetic prospecting. If so, and if drug development firms are increasingly turning to rational drug design and other methods of drug development, then the basic premise of the United Nations Convention on Biological Diversity (CBD) - that conserving biodiversity is a function of developing genetic resources for pharmaceutical and biotechnological application - rests on tenuous grounds, not to mention the entire discourse on bioprospecting as a sustainability mechanism. If there are few future monetary benefits from biodiversity, if indeed its marginal value is close to zero, then expressed conservation objectives could further succumb to short-term, economically beneficial but environmentally destructive agricultural and timber harvesting practices....