Ok I'm gonna clear things a bit
First, I encourage anybody to only allow one upload slot per IP (Options -> Tranfers -> Places (might be a little different with LW/FW in english, lol) )
Normally LW should only upload a single file once to an IP, not twice or three times. The upload code in LW is a mess and should be worked on in the next year. Allowing one upload per person will fix that bug and allows spreading files to more people.
Next, the upload variety. We have been discussing that a lot both with LW programmers and others. It points out that putting a limit on upload slots for one file (ie you can't upload a file to more than three people in order to share other rare files) would slow network and would be difficult to implement correctly. If a file is taking all your upload slots, it is a popular file and other people are probably already sharing that file, so you should unshare that file if you want to benefit to the network by sharing other files. That's what I do
Why some days some files are more in demand? Well, because some days there is a download mesh occuring around one file you share. That means that even though your aren't appearing as a result for one search, other hosts know you have the file. The download mesh in Gnutella is exactly that, as soon as somebody get a request for a file it uploads, it will send alternative host addresses that share that particular file. That means you are into a download mesh and for popular files, instantly your upload queue will be filled for one file. That system helps finishing downloads and getting more download speed, but it also lowers the amount of file available on Gnutella. That explains why Gnutella is a fast distribution content system while another like eDonkey is slow but instead allows a much more broad content library to be shared and downloaded.
Ciao