"(for instance: you need to click the tabs in the launcher menu instead of just mousing over them, which is unpleasant)"
we have a policy of having everything go upstream unless there is very good reason. I just checked and the issue you say is not true (now).
"Is there any reason to use Kubuntu instead of just about any other KDE based distro?"
We believe KDE to be the best technology and therefore way to take over the world. Other distros will fill in gaps in KDE's offering with non-KDE apps but we are much more reluctant to do that. If you are interested in having short term solutions go with other distros which ship non-KDE web browsers etc.