From the same version of Apache-Tomcat (6.0.18) web.xml config file:
<!-- classpath What class path should I use while compiling -->
<!-- generated servlets? [Created dynamically -->
<!-- based on the current web application] -->
<!-- -->
You are setting CLASSPATH as an OS environment variable. Tomcat wants/has it's own and you set it within the engine, or let the engine take care of it for you.
interesting thanks - will investigate that