One problem with selenium (using either IDE, RC or Webdriver) is that you don't know what actually happened during the execution.
The selenium result would give some hints, e.g. if the
verifyTextPresent failed then you know there is a missing text during execution. But you would like to know what actually appears on that screen? Did the page show up an error page? Or just the value is changed, or probably the server died?
The nature of automated testing is that you are unlikely to watch the executions all the time. Otherwise it's pointless to do the automation.
Read more Painful Selenium Testing – Part 4 – Firefox Screenshot