I am one of "those" parasites - and I use a site downloader quite frequently, for a number of reasons. Now, to be fair I don't use it when I suspect someone will have bandwidth problems to try and be fair to them.

As to why? Lot's of reasons:

• Many websites are maintained badly. The content disappears or gets replaced with no later ability to get to it. Often having a link to the site is later useless. I grab 'em, slam 'em onto a CD and file them away. Nice and easy.
• It is simply much easier/faster for me to evaluate the contents on my own drives.
• I know in advance I want it all. For instance I often grab entire modeling portfolios of models. I know in advance I want all the images... and this is a fast way to get them.
• The site is designed so badly that I have no urge to try and navigate it by hand. I let the software grab it all them I look at the images and files with better tools than a web browser.

I always know the site I am grabbing, and why I am doing it. I also know lots of others who use these tools and the are similar, they know they want the site, so they go get it.

Alternately, I also use a download manager. When I go to grab a file is makes multiple connections to the server and grabs that way. My throughput goes WAY up and the net amount of transfer is the same.

Now, can these things be used irresponsibly? Sure.Are they always used that way? Nope.