a really useful discussion...so it seems I'm not the only one who thinks that pinned tabs should be protected, even better than only closing it.
I left slimjet because of this behavior (well, this was not only because of that :p )
I see pinned tabs like a way to keep track of my favorite websites (those I cannot "live without"). We should not be able to close it, even more to overwrite it (I'm always opening url/fav in a new tab, so I'm not concerned with overwriting). And closing that kind of tab is as easy as : unpin > close.
pinning a tab is not a way to take less space (IMHO people using it for this did not understood what it was designed for)
this is my 2 cents on the subject, I really hope that you will consider, at least, adding an option to keep protected mode for pinned tabs (close, and why not...overwrite). But for now, I will stay in 2.1.9.50
thanks for your answer and reading.