On their way, the bookmarks get corrupted (especially non-English page titles, but also some URLs, which most times get some extra whitespaces). When importing bookmarks from iCab via IE into Safari, this always happens on my machine.
You won't see (all of) this when using Safari's built-in bookmark manager, since the browser loads even malformed bookmarks, but you will see them when editing the associated preferences-file. In my case, there were over 300 errors according to the validator.
After I edited the bookmark XML file by hand to make it comply, Safari was way less instable (crashed spontaneously when organizing bookmarks, sometimes seemingly unassociated) and loaded faster.
[Editor's note: This is the first I've heard of a potential problem with corrupted bookmarks causing Safari crashes -- can anyone add any evidence one way or the other?]

