When one set a build parameter to an empty value, ant build steps end up with a broken command line. I can't see what happens with the command line exactly but seems like the next build parameter ends up as a value to the -D<param>=
One would want to have such setup for example with the JAVA_OPTS environment variable. It would normally be unset but set to some value for debugging purposes or depending on the slave the build is executing on.
Of course workarounds exist but parameters are a great convenience.