prove has many benefits that people don't always appreciate. It's easy to do recursive tests, shuffle tests, run all tests in a particular directory, etc. However, with Test::Harness 3.0, you get much more. You can:

• Switch in your own test harness
• Have greater control over what you do and do not see (quiet output, only failures, etc.)
• Get colored test output
• Run tests in parallel
• Create TAP archives
• ... or any other custom test behavior you need.

In short, the current prove is useful, the upcoming version is even more useful.

As for adding the -d switch, that becomes very handy when you think there might be a problem with test/code interaction (ever seen code fail in the test suite but not when run directly?).