The apps simply try to make the best use of the memory available, for example they could use it for caching. It would be bad if they weren't using up all the memory available.
That said, there might be the occasional rogue app or web site, which should be uninstalled or avoided.
I guess Rogue is a term to describe both malicious or poorly coded sites. As a rule of thumb, if scrolling isn't perfectly instant, then a site is "rogue". I found a recent example of a unintentional rogue website: it was a tumblr blog using a preview service that would extract opengraph meta data and display images as a caption to the link. One of the link was an article on a news website where the og:image was incorrectly pointing on a 3MB jpg. The whole tumblr blog would be totally unresponsive to scrolling due to a 50x50 image with a 4096x2048 src.
This was absolutely unintentional and involves at least two intermediates over which the original blogger had no control whatsoever.
Link to the page in question: http://fh2012.tumblr.com/page/6
It so happens it's the tumblr blog of the current favorite to the French Presidential election, and it was the first post so I noticed it :P
I would have preferred a web in which scrolling (and certain other operations, like the Back button) was always instant regardless of how the page is coded, and consequently I did not have to watch out for rogue pages or sites.
ADDED. Or at least a web in which there were fewer ways for a page to go unresponsive, which would have made it easier for readers and writers to anticipate responsiveness problems.
That said, there might be the occasional rogue app or web site, which should be uninstalled or avoided.