The topic of this thread could mean many different people including those who choose not to share at all. 'Leeches' who suck your files from you but never give anything back.
On this occasion this topic is referring to a specific type of sharer. Those who share 'some' files, but remove them from their library as soon as someone starts to download them, or as soon as possible after they download them.

I've noticed this in recent months I've browsed a host, generally from upload window. I've selected one of their files to download. It immediately goes to 'Need More Sources' status. Same thing happens with the next file & the next. I repeatedly cancel the download & reselect from browse host results. Same result each time.

I discovered why this was happening after I downloaded two & a half small files from one host, I sent them a Chat message. They told me they were removing these particular files from their library. They were uploading from me whilst this was happening. I did not ban this host due to their honesty. I have found some people remove all the files they initially show in their share library after browsing them.

The cheek of people removing files from their Shares whilst they are uploading from me!!! I very frequently Browse hosts uploading me when I am able to in hope they share similar files I am seeking.

I ban each host I find who does this. They will never get any more of my multiple thousands of files again if I can help it. I would love to pass a blacklist of these ip addresses around to all other people who do genuinely share files. However forum rules does not permit me. There are other means of 'sharing' this information however if I get annoyed enough about it.

Of course they could be DDOS'd with the assistance of people's friends. woot! I'd hate to think of anybody doing that.