I have okular installed here (Fedora 12) and I cannot reproduce the behavior you describe.
One problem is that, most of the time, when I'm scrolling through a PDF, I want to read the pages from the top down (i.e. scroll forward through the file). However, in order to scroll forward by dragging the main page with the left mouse button, the mouse cursor itself actually has to move up in order for the page content to move down. So my mouse cursor never hits the bottom of the screen like you describe, unless I'm scrolling backwards, which happens very rarely. When I scroll forward, the cursor hits the top of the screen, and when it hits the top, it certainly doesn't automatically wrap the cursor to the bottom.
Since I cannot reproduce this behavior, I have to make certain assumptions about what you mean. Assuming you meant that the mouse cursor wraps from top to bottom, I can see how it would be a worthwhile option, but I would never use it myself. Most of my pdf reading occurs on a laptop, with a touchpad, in which case dragging the page is worse than useless -- it requires holding down a button as well as moving a finger along the touchpad, whereas the scrollwheel is built in to the touchpad and only requires moving a finger along the touchpad, and thus involves strictly less work. The only time I use dragging is for fine (pixel-level) scrolling control that cannot be achieved with the scroll wheel, but in such cases wraparound is unnecessary.
In addition to the lack of utility, my own opinion is that the bottom of the screen should be an absolute boundary to movement, not an invitation to wrap the cursor around to the top of the screen, no matter how worthy the justification may be. Moreover, if the PDF is displayed in a window, rather than full screen, then automatic cursor wraparound would be even more jarring, as it would jump from the top of the window to the bottom of the window rather than the top of the screen to the bottom of the screen.
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