Version: (using KDE 4.1.2) Installed from: Ubuntu Packages KDE4 should adopt the close button on tab option as the default. The close button on tab fits with the ideal of "doing things in place" (contextually). Although there was a lot of discussion about the concept when it hit mainstream users via Firefox, I think it has now had exposure to enough users to a) be proven as a good design decision and b) not be a surprise change which is hard to adapt to.
Tabs are meant to switch to switch documents, spaces, etc.; not to close them. It is not rare to accidentally close a tab when the intention was to switch to one besides it. Even if you are very careful, the downside is that your mind has to process a bit more information. Yes, I am lazy. The biggest pro (for me) of having the close button in one place is that it *always* is in that place. As I usually use the keyboard (or the scroll wheel) to switch tabs I forget where in the tab bar the it is located. So, if I want to close it with the mouse for any reason, I know immediately where to go. This may not be convincing but it is my attempt to keep what, I think, is a good default.
I changed defaults to have close button on tabs (trunk only, r909806). Discussion at #kde-devel revealed that people have very different ways to handle tabs but there were no objections about this change. Ok'd by dfaure.